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64-702: Fis was first discovered for its role in stimulating Gin catalyzed inversion of the G segment of phage Mu genome. Fis was originally identified as the f actor for i nversion s timulation of the homologous Hin and Gin site-specific DNA recombinases of Salmonella and phage Mu, respectively. This small, basic, DNA-bending protein has recently been shown to function in many other reactions including phage lambda site-specific recombination, transcriptional activation of rRNA and tRNA operons, repression of its own synthesis, and oriC-directed DNA replication. Cellular concentrations of Fis vary tremendously under different growth conditions which may have important regulatory implications for

128-499: A DNA binding site called a promoter region before RNAP can initiate the DNA unwinding at that position. RNAP not only initiates RNA transcription, it also guides the nucleotides into position, facilitates attachment and elongation , has intrinsic proofreading and replacement capabilities, and termination recognition capability. In eukaryotes , RNAP can build chains as long as 2.4 million nucleotides. RNAP produces RNA that, functionally,

192-449: A beta (β) subunit of 150 kDa, a beta prime subunit (β′) of 155 kDa, and a small omega (ω) subunit. A sigma (σ) factor binds to the core, forming the holoenzyme. After transcription starts, the factor can unbind and let the core enzyme proceed with its work. The core RNA polymerase complex forms a "crab claw" or "clamp-jaw" structure with an internal channel running along the full length. Eukaryotic and archaeal RNA polymerases have

256-458: A distinct subset of RNA: The 2006 Nobel Prize in Chemistry was awarded to Roger D. Kornberg for creating detailed molecular images of RNA polymerase during various stages of the transcription process. In most prokaryotes , a single RNA polymerase species transcribes all types of RNA. RNA polymerase "core" from E. coli consists of five subunits: two alpha (α) subunits of 36  kDa ,

320-492: A few single-subunit RNA polymerases (ssRNAP) from phages and organelles. The other multi-subunit RNAP lineage formed all of the modern cellular RNA polymerases. In bacteria , the same enzyme catalyzes the synthesis of mRNA and non-coding RNA (ncRNA) . RNAP is a large molecule. The core enzyme has five subunits (~400 kDa ): In order to bind promoters, RNAP core associates with the transcription initiation factor sigma (σ) to form RNA polymerase holoenzyme. Sigma reduces

384-419: A group consisting of 10 PAPs was identified through biochemical methods, which was later extended to 12 PAPs. Chloroplast also contain a second, structurally and mechanistically unrelated, single-subunit RNAP ("nucleus-encoded polymerase, NEP"). Eukaryotic mitochondria use POLRMT (human), a nucleus-encoded single-subunit RNAP. Such phage-like polymerases are referred to as RpoT in plants. Archaea have

448-516: A multi-subunit RNAP ("PEP, plastid-encoded polymerase"). Due to its bacterial origin, the organization of PEP resembles that of current bacterial RNA polymerases: It is encoded by the RPOA, RPOB, RPOC1 and RPOC2 genes on the plastome, which as proteins form the core subunits of PEP, respectively named α, β, β′ and β″. Similar to the RNA polymerase in E. coli , PEP requires the presence of sigma (σ) factors for

512-460: A native turn region signals the folding cascade to start, where a native turn is one that is present in the final folded structure. In the folding of overall proteins, the turn may originate not in the native turn region but in the C-strand of the beta-hairpin. This turn then propagates through the C-strand (the beta strand leading to C-terminus) until it reaches the native turn region. Sometimes

576-497: A nutritional upshift and is important in the physiological roles Fis plays in the cell. It is a global regulatory protein in Escherichia coli that activates ribosomal RNA ( rRNA ) transcription by binding to three upstream activation sites of the rRNA promoter and enhances transcription 5 to 10 fold in vivo . Fis overexpression results in different effects on cell growth depending on nutrient conditions. The Fis nucleoid protein

640-527: A proline leading up to the native turn region) and less conformational options. The initial turn formation takes place in about 1 μs. Once the initial turn has been established, two mechanisms have been proposed as to how the rest of the beta-hairpin folds: a hydrophobic collapse with side-chain level rearrangements, or the more accepted zipper-like mechanism. The β-hairpin loop motif can be found in many macromolecular proteins. However, small and simple β-hairpins can exist on their own as well. To see this clearly,

704-556: A result, the 8 bp DNA-RNA hybrid in the transcription complex shifts to a 4 bp hybrid. These last 4 base pairs are weak A-U base pairs, and the entire RNA transcript will fall off the DNA. Transcription termination in eukaryotes is less well understood than in bacteria, but involves cleavage of the new transcript followed by template-independent addition of adenines at its new 3′ end, in a process called polyadenylation . Given that DNA and RNA polymerases both carry out template-dependent nucleotide polymerization, it might be expected that

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768-497: A similar core structure and work in a similar manner, although they have many extra subunits. All RNAPs contain metal cofactors , in particular zinc and magnesium cations which aid in the transcription process. Control of the process of gene transcription affects patterns of gene expression and, thereby, allows a cell to adapt to a changing environment, perform specialized roles within an organism, and maintain basic metabolic processes necessary for survival. Therefore, it

832-476: A single type of RNAP, responsible for the synthesis of all RNA. Archaeal RNAP is structurally and mechanistically similar to bacterial RNAP and eukaryotic nuclear RNAP I-V, and is especially closely structurally and mechanistically related to eukaryotic nuclear RNAP II. The history of the discovery of the archaeal RNA polymerase is quite recent. The first analysis of the RNAP of an archaeon was performed in 1971, when

896-522: A structural function. Archaeal RNAP subunit previously used an "RpoX" nomenclature where each subunit is assigned a letter in a way unrelated to any other systems. In 2009, a new nomenclature based on Eukaryotic Pol II subunit "Rpb" numbering was proposed. Orthopoxviruses and some other nucleocytoplasmic large DNA viruses synthesize RNA using a virally encoded multi-subunit RNAP. They are most similar to eukaryotic RNAPs, with some subunits minified or removed. Exactly which RNAP they are most similar to

960-470: A template instead of DNA). This occurs in negative strand RNA viruses and dsRNA viruses , both of which exist for a portion of their life cycle as double-stranded RNA. However, some positive strand RNA viruses , such as poliovirus , also contain RNA-dependent RNAP. RNAP was discovered independently by Charles Loe, Audrey Stevens , and Jerard Hurwitz in 1960. By this time, one half of

1024-413: A three-residue, and class 4 with a four-residue. Class 5 does not exist as that primary hairpin is already defined in class 1. Pi This classification scheme not only accounts for various degrees of hydrogen bonding, but also says something about the biological behavior of the hairpin. Single amino acid replacements may destroy a particular hydrogen bond, but will not unfold the hairpin or change its class. On

1088-478: A transcriptional repressor of ''mom'' promoter . There is data that shows Fis mediates its repressive effect by denying access to RNA polymerase at the mom promoter. combined A repressive effect of Fis and previously characterized negative regulatory factors could be responsible to keep the gene silenced most of the time. In addition to bringing about overall downregulation of the Mu genome , it also ensures silencing of

1152-457: Is bacteriophage T7 RNA polymerase . ssRNAPs cannot proofread. B. subtilis prophage SPβ uses YonO, a homolog of the β+β′ subunits of msRNAPs to form a monomeric (both barrels on the same chain) RNAP distinct from the usual "right hand" ssRNAP. It probably diverged very long ago from the canonical five-unit msRNAP, before the time of the last universal common ancestor . Other viruses use an RNA-dependent RNAP (an RNAP that employs RNA as

1216-417: Is a topic of debate. Most other viruses that synthesize RNA use unrelated mechanics. Many viruses use a single-subunit DNA-dependent RNAP (ssRNAP) that is structurally and mechanistically related to the single-subunit RNAP of eukaryotic chloroplasts (RpoT) and mitochondria ( POLRMT ) and, more distantly, to DNA polymerases and reverse transcriptases . Perhaps the most widely studied such single-subunit RNAP

1280-639: Is adjacent to the C-terminus of the next), and linked by a short loop of two to five amino acids . Beta hairpins can occur in isolation or as part of a series of hydrogen bonded strands that collectively comprise a beta sheet . Researchers such as Francisco Blanco et al. have used protein NMR to show that beta-hairpins can be formed from isolated short peptides in aqueous solution, suggesting that hairpins could form nucleation sites for protein folding . Beta hairpins were originally categorized solely by

1344-402: Is an enzyme that catalyzes the chemical reactions that synthesize RNA from a DNA template. Using the enzyme helicase , RNAP locally opens the double-stranded DNA so that one strand of the exposed nucleotides can be used as a template for the synthesis of RNA, a process called transcription . A transcription factor and its associated transcription mediator complex must be attached to

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1408-522: Is an additional subunit dubbed Rpo13; together with Rpo5 it occupies a space filled by an insertion found in bacterial β′ subunits (1,377–1,420 in Taq ). An earlier, lower-resolution study on S. solfataricus structure did not find Rpo13 and only assigned the space to Rpo5/Rpb5. Rpo3 is notable in that it's an iron–sulfur protein . RNAP I/III subunit AC40 found in some eukaryotes share similar sequences, but does not bind iron. This domain, in either case, serves

1472-475: Is cotranscribed with the upstream dusB gene encoding a tRNA-modifying enzyme. DusB protein levels are very low even under conditions when there is high transcription of the operon and high levels of Fis protein. Fis has been deemed a bacterial chromatin architectural protein. Besides modulating chromatin architecture, it is known to influence numerous promoters of E. coli and several other bacteria. Both in vivo and in vitro studies indicate that Fis acts as

1536-544: Is differentiated by its fast increase in synthesis rates following nutrient upshifts and its abundance in rapidly growing E. coli cells. Fis has been known to activate ribosomal RNA transcription, as well other genes. It has a direct role in upstream activation of rRNA promoters. Fis binds to a recombinational enhancer sequence that is required to stimulate hin -mediated DNA inversion. It has also been shown to prevent initiation of DNA replication from oriC . It has been shown that sequences from 32 to 94 nucleotides upstream of

1600-458: Is either for protein coding , i.e. messenger RNA (mRNA); or non-coding (so-called "RNA genes"). Examples of four functional types of RNA genes are: RNA polymerase is essential to life, and is found in all living organisms and many viruses . Depending on the organism, a RNA polymerase can be a protein complex (multi-subunit RNAP) or only consist of one subunit (single-subunit RNAP, ssRNAP), each representing an independent lineage. The former

1664-481: Is found in bacteria , archaea , and eukaryotes alike, sharing a similar core structure and mechanism. The latter is found in phages as well as eukaryotic chloroplasts and mitochondria , and is related to modern DNA polymerases . Eukaryotic and archaeal RNAPs have more subunits than bacterial ones do, and are controlled differently. Bacteria and archaea only have one RNA polymerase. Eukaryotes have multiple types of nuclear RNAP, each responsible for synthesis of

1728-419: Is hardly surprising that the activity of RNAP is long, complex, and highly regulated. In Escherichia coli bacteria, more than 100 transcription factors have been identified, which modify the activity of RNAP. RNAP can initiate transcription at specific DNA sequences known as promoters . It then produces an RNA chain, which is complementary to the template DNA strand. The process of adding nucleotides to

1792-424: Is important in the adaptation of P. putida during colonization of plant roots by promoting biofilm formation when the migration of bacteria is no longer advantageous. It was demonstrated that Fis is essential for the stability of the linear plasmid pDSIUDi and affects the motility of S. Typhi . Fis buffers decrease of negative supercoiling in tyrT and rrnA expression. The upstream FIS binding site of rrnA

1856-407: Is largely shut off, and intracellular Fis levels decrease as a function of cell division. Fis synthesis also transiently increases when exponentially growing cells are shifted to a richer medium. The magnitude of the peak of Fis synthesis appears to reflect the extent of the nutritional upshift. fis mRNA levels closely resemble the protein expression pattern, suggesting that regulation occurs largely at

1920-581: Is required for this and it's probable that FIS enables local DNA curvature. See Travers and Muskhelishvili 2005 for more detail. Beta hairpin The beta hairpin (sometimes also called beta-ribbon or beta-beta unit ) is a simple protein structural motif involving two beta strands that look like a hairpin . The motif consists of two strands that are adjacent in primary structure , oriented in an antiparallel direction (the N-terminus of one sheet

1984-581: The Pin1 Domain protein is shown to the left as an example. Proteins that are β-sheet rich, also called WW domains , function by adhering to proline-rich and/or phosphorylated peptides to mediate protein–protein interactions . The "WW" refers to two tryptophan (W) residues that are conserved within the sequence and aid in the folding of the β-sheets to produce a small hydrophobic core. These tryptophan residues can be seen below (right) in red. This enzyme binds its ligand through van der Waals forces of

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2048-473: The fis AUG are responsible for increasing fis lacZ translation reporter activities over 100 fold. Within this region, an AU sequence element centered 35 nucleotides upstream of the fis AUG increases fis translation by as much as 15 fold. Formation of a supposed RNA secondary structure element beginning 50 nucleotides upstream of the AUG also positively affects fis translation by up to 10 fold. The fis gene

2112-479: The residue interactions leading up to the native turn region are too strong, causing reverse propagation. However, once the native turn does form, interactions between prolines and tryptophan residues (seen in image at right) in the region help to stabilize the turn, preventing "roll back" or dissolution. Researchers believe that turns do not originate in the N-strand, due to increased rigidity (often caused by

2176-525: The rho factor , which destabilizes the DNA-RNA heteroduplex and causes RNA release. The latter, also known as intrinsic termination , relies on a palindromic region of DNA. Transcribing the region causes the formation of a "hairpin" structure from the RNA transcription looping and binding upon itself. This hairpin structure is often rich in G-C base-pairs, making it more stable than the DNA-RNA hybrid itself. As

2240-421: The " transcription bubble ". Supercoiling plays an important part in polymerase activity because of the unwinding and rewinding of DNA. Because regions of DNA in front of RNAP are unwound, there are compensatory positive supercoils. Regions behind RNAP are rewound and negative supercoils are present. RNA polymerase then starts to synthesize the initial DNA-RNA heteroduplex, with ribonucleotides base-paired to

2304-419: The DNA template strand. As transcription progresses, ribonucleotides are added to the 3′ end of the RNA transcript and the RNAP complex moves along the DNA. The characteristic elongation rates in prokaryotes and eukaryotes are about 10–100 nts/sec. Aspartyl ( asp ) residues in the RNAP will hold on to Mg ions, which will, in turn, coordinate the phosphates of the ribonucleotides. The first Mg will hold on to

2368-524: The DNA template. This pauses transcription. The polymerase then backtracks by one position and cleaves the dinucleotide that contains the mismatched nucleotide. In the RNA polymerase this occurs at the same active site used for polymerization and is therefore markedly different from the DNA polymerase where proofreading occurs at a distinct nuclease active site. The overall error rate is around 10 to 10 . In bacteria, termination of RNA transcription can be rho-dependent or rho-independent. The former relies on

2432-425: The RNA strand is known as elongation; in eukaryotes, RNAP can build chains as long as 2.4 million nucleotides (the full length of the dystrophin gene). RNAP will preferentially release its RNA transcript at specific DNA sequences encoded at the end of genes, which are known as terminators . Products of RNAP include: RNAP accomplishes de novo synthesis . It is able to do this because specific interactions with

2496-487: The RNAP from the extreme halophile Halobacterium cutirubrum was isolated and purified. Crystal structures of RNAPs from Sulfolobus solfataricus and Sulfolobus shibatae set the total number of identified archaeal subunits at thirteen. Archaea has the subunit corresponding to Eukaryotic Rpb1 split into two. There is no homolog to eukaryotic Rpb9 ( POLR2I ) in the S. shibatae complex, although TFS (TFIIS homolog) has been proposed as one based on similarity. There

2560-523: The active center stabilizes the elongation complex. However, promoter escape is not the only outcome. RNA polymerase can also relieve the stress by releasing its downstream contacts, arresting transcription. The paused transcribing complex has two options: (1) release the nascent transcript and begin anew at the promoter or (2) reestablish a new 3′-OH on the nascent transcript at the active site via RNA polymerase's catalytic activity and recommence DNA scrunching to achieve promoter escape. Abortive initiation ,

2624-581: The advantageous but potentially lethal mom gene. Fis as a critical regulator of capsule expression. Fis is also involved in the regulation of a range of genes in bacterial species such as P. multocida , Enteroaggregative Escherichia coli, similar organisms. Some of these genes include important virulence factors. The role of fis is well studied in E. coli, but its role in pseudomonads has only been examined briefly. Recent studies in Enterobacteriaceae have shown that fis positively regulates

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2688-449: The affinity of RNAP for nonspecific DNA while increasing specificity for promoters, allowing transcription to initiate at correct sites. The complete holoenzyme therefore has 6 subunits: β′βα and α ωσ (~450 kDa). Eukaryotes have multiple types of nuclear RNAP, each responsible for synthesis of a distinct subset of RNA. All are structurally and mechanistically related to each other and to bacterial RNAP: Eukaryotic chloroplasts contain

2752-523: The bacterial chromosome structure and the initiation of DNA replication. It is a nucleoid-associated protein in Escherichia coli that is abundant during early exponential growth in rich medium but is in short supply during stationary phase. When stationary-phase cells are subcultured into a rich medium, Fis levels increase from less than 100 to over 50,000 copies per cell prior to the first cell division. As cells enter exponential growth, nascent synthesis

2816-410: The conserved tryptophans and the proline-rich areas of the ligand. Other amino acids can then associate with the hydrophobic core of the β-hairpin structure to enforce secure binding. It is also common to find proline residues within the actual loop portion of the β-hairpin, since this amino acid is rigid and contributes to the "turn" formation. These proline residues can be seen as red side chains in

2880-612: The core promoter region containing the −35 and −10 elements (located before the beginning of sequence to be transcribed) and also, at some promoters, the α subunit C-terminal domain recognizing promoter upstream elements. There are multiple interchangeable sigma factors, each of which recognizes a distinct set of promoters. For example, in E. coli , σ is expressed under normal conditions and recognizes promoters for genes required under normal conditions (" housekeeping genes "), while σ recognizes promoters for genes required at high temperatures (" heat-shock genes "). In archaea and eukaryotes,

2944-472: The flagellar movement of bacteria. Observations in Pseudomonas putida demonstrate fis reduced the migration of P. putida onto the apices of barley roots and thereby the competitiveness of bacteria on the roots. It was also observed that the overexpression of fis drastically reducing swimming motility and facilitated the formation of P. putida biofilm. It is possible that the elevated expression of Fis

3008-486: The functions of the bacterial general transcription factor sigma are performed by multiple general transcription factors that work together. The RNA polymerase-promoter closed complex is usually referred to as the " transcription preinitiation complex ." After binding to the DNA, the RNA polymerase switches from a closed complex to an open complex. This change involves the separation of the DNA strands to form an unwound section of DNA of approximately 13 bp, referred to as

3072-545: The image of the Pin1 WW domain below (left). The design of peptides that adopt β-hairpin structure (without relying on metal binding, unusual amino acids, or disulfide crosslinks) has made significant progress and yielded insights into protein dynamics. Unlike α-helices , β-hairpins are not stabilized by a regular hydrogen bonding pattern. As a result, early attempts required at least 20–30 amino acid residues to attain stable tertiary folds of β-hairpins. However, this lower limit

3136-429: The initiating nucleotide hold RNAP rigidly in place, facilitating chemical attack on the incoming nucleotide. Such specific interactions explain why RNAP prefers to start transcripts with ATP (followed by GTP, UTP, and then CTP). In contrast to DNA polymerase , RNAP includes helicase activity, therefore no separate enzyme is needed to unwind DNA. RNA polymerase binding in bacteria involves the sigma factor recognizing

3200-482: The initiation complex. During the promoter escape transition, RNA polymerase is considered a "stressed intermediate." Thermodynamically the stress accumulates from the DNA-unwinding and DNA-compaction activities. Once the DNA-RNA heteroduplex is long enough (~10 bp), RNA polymerase releases its upstream contacts and effectively achieves the promoter escape transition into the elongation phase. The heteroduplex at

3264-456: The loop sequence but also signal the end of the loop, thus defining this hairpin as a three-residue loop. This single hydrogen bond is then removed to create the tertiary hairpin; a five-residue loop with doubly bound residues. This pattern continues indefinitely and defines all beta hairpins within the class. Class 2 follows the same pattern beginning with a two-residue loop with terminating residues that share two hydrogen bonds. Class 3 begins with

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3328-434: The mechanism through which micro-domains fold can help to shed light onto the folding patterns of whole proteins . Studies of a beta hairpin called chignolin (see Chignolin on Proteopedia ) have uncovered a stepwise folding process that drives beta-hairpin folding. This hairpin has sequence features similar to over 13,000 known hairpins, and thus may serve as a more general model for beta hairpin formation. The formation of

3392-458: The number of amino acid residues in their loop sequences, such that they were named one-residue, two-residue, etc. This system, however, is somewhat ambiguous as it does not take into account whether the residues that signal the end of the hairpin are singly or doubly hydrogen bonded to one another. An improved means of classification has since been proposed by Milner-White and Poet. Beta hairpins are broken into four distinct classes as depicted in

3456-408: The other hand, amino acid insertions and deletions will have to unfold and reform the entire beta strand in order to avoid a beta bulge in the secondary structure. This will change the class of the hairpin in the process. As substitutions are the most common amino acid mutations, a protein could potentially undergo a conversion without affecting the functionality of the beta hairpin. Understanding

3520-410: The physiological role of Fis in these different reactions. Structurally, Fis folds into four α-helices (A–D) and a β-hairpin . Helices A and B provide the contacts between Fis monomers, facilitating dimer formation, whereas the C and D helices form a helix-turn-helix motif that is essential for DNA binding. Fis is a very important small nucleotide-associated protein which plays a role in affecting

3584-436: The publication's Figure 1. Each class begins with the smallest possible number of loop residues and progressively increases the loop size by removing hydrogen bonds in the beta sheet. The primary hairpin of class 1 is a one-residue loop where the bound residues share two hydrogen bonds. One hydrogen bond is then removed to create a three-residue loop, which is the secondary hairpin of class 1. Singly bound residues are counted in

3648-450: The recognition of its promoters, containing the -10 and -35 motifs. Despite the many commonalities between plant organellar and bacterial RNA polymerases and their structure, PEP additionally requires the association of a number of nuclear encoded proteins, termed PAPs (PEP-associated proteins), which form essential components that are closely associated with the PEP complex in plants. Initially,

3712-500: The template DNA strand according to Watson-Crick base-pairing interactions. As noted above, RNA polymerase makes contacts with the promoter region. However these stabilizing contacts inhibit the enzyme's ability to access DNA further downstream and thus the synthesis of the full-length product. In order to continue RNA synthesis, RNA polymerase must escape the promoter. It must maintain promoter contacts while unwinding more downstream DNA for synthesis, "scrunching" more downstream DNA into

3776-525: The transcriptional level. Two RNA polymerase-binding sites and at least six high-affinity Fis-binding sites are present in the fis promoter region. Expression of this fis operon is negatively regulated by Fis in vivo and purified Fis can prevent stable complex formation by RNA polymerase at the fis promoter in vitro. However, autoregulation only partially accounts for the expression pattern of Fis. Fluctuations in Fis levels have been shown to serve as an early signal of

3840-642: The turn are replaced by azobenzene , which can be induced to switch from the trans to the cis conformation by light at 360 nm. When the azobenzene moiety is in the cis conformation, the amino acid residues align correctly to adopt a β-hairpin formation. However, the trans conformation does not have proper turn geometry for the β-hairpin. This phenomenon can be used to investigate peptide conformational dynamics with femtosecond absorption spectroscopy. RNA polymerase In molecular biology , RNA polymerase (abbreviated RNAP or RNApol ), or more specifically DNA-directed/dependent RNA polymerase ( DdRP ),

3904-466: The two types of enzymes would be structurally related. However, x-ray crystallographic studies of both types of enzymes reveal that, other than containing a critical Mg ion at the catalytic site, they are virtually unrelated to each other; indeed template-dependent nucleotide polymerizing enzymes seem to have arisen independently twice during the early evolution of cells. One lineage led to the modern DNA polymerases and reverse transcriptases, as well as to

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3968-422: The unproductive cycling of RNA polymerase before the promoter escape transition, results in short RNA fragments of around 9 bp in a process known as abortive transcription. The extent of abortive initiation depends on the presence of transcription factors and the strength of the promoter contacts. The 17-bp transcriptional complex has an 8-bp DNA-RNA hybrid, that is, 8 base-pairs involve the RNA transcript bound to

4032-479: The α-phosphate of the NTP to be added. This allows the nucleophilic attack of the 3′-OH from the RNA transcript, adding another NTP to the chain. The second Mg will hold on to the pyrophosphate of the NTP. The overall reaction equation is: Unlike the proofreading mechanisms of DNA polymerase those of RNAP have only recently been investigated. Proofreading begins with separation of the mis-incorporated nucleotide from

4096-605: Was reduced to 12 amino acids by the stability gains conferred by the incorporation of tryptophan-tryptophan cross-strand pairs. Two nonhydrogen-bonding tryptophan pairs have been shown to interlock in a zipper-like motif, stabilizing the β-hairpin structure while still allowing it to remain water-soluble . The NMR structure of a tryptophan zipper (trpzip) β-peptide shows the stabilizing effect of favorable interactions between adjacent indole rings. The synthesis of trpzip β-hairpin peptides has incorporated photoswitches that facilitate precise control over folding. Several amino acids in

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