The homing endonucleases are a collection of endonucleases encoded either as freestanding genes within introns , as fusions with host proteins, or as self-splicing inteins . They catalyze the hydrolysis of genomic DNA within the cells that synthesize them, but do so at very few, or even singular, locations. Repair of the hydrolyzed DNA by the host cell frequently results in the gene encoding the homing endonuclease having been copied into the cleavage site, hence the term 'homing' to describe the movement of these genes. Homing endonucleases can thereby transmit their genes horizontally within a host population, increasing their allele frequency at greater than Mendelian rates.
8-404: Intron-encoded endonuclease I-Sce I is a homing endonuclease . The enzyme is used in biotechnology as a meganuclease . It recognises an 18-base pair sequence TAGGGATAACAGGGTAAT and leaves a 4 base pair 3' hydroxyl overhang. It is a rare cutting endonuclease . Statistically an 18-bp sequence will occur once in every 6.9*10 base pairs (a frequency of 1 in 4). This sequence does not normally occur in
16-482: A human or mouse genome. I-SceI is coded by introns . It is present in the mitochondria of yeast Saccharomyces cerevisiae . This molecular or cell biology article is a stub . You can help Misplaced Pages by expanding it . Homing endonuclease Although the origin and function of homing endonucleases is still being researched, the most established hypothesis considers them as selfish genetic elements , similar to transposons , because they facilitate
24-399: A laboratory. Next, a three-letter name is derived from the binominal name of the organism, taking one uppercase letter from the genus name and two lowercase letters from the specific name. (Some mixing is usually done for hybrid enzymes.) Finally, a Roman numeral distinguishes different enzymes found in the same organism: Homing endonucleases differ from Type II restriction enzymes in
32-453: Is called "homing". Homing endonucleases are always indicated with a prefix that identifies their genomic origin, followed by a hyphen: "I-" for homing endonucleases encoded within an intron, "PI-" (for "protein insert") for those encoded within an intein. Some authors have proposed using the prefix "F-" ("freestanding") for viral enzymes and other natural enzymes not encoded by introns nor inteins, and "H-" ("hybrid") for enzymes synthesized in
40-461: The endonuclease (the HEG, "homing endonuclease gene") is located within the recognition sequence which the enzyme cuts, thus interrupting the homing endonuclease recognition sequence and limiting DNA cutting only to sites that do not (yet) carry the HEG. Prior to transmission, one allele carries the gene (HEG ) while the other does not (HEG ), and is therefore susceptible to being cut by the enzyme. Once
48-424: The enzyme is synthesized, it breaks the chromosome in the HEG allele, initiating a response from the cellular DNA repair system. The damage is repaired using recombination , taking the pattern of the opposite, undamaged DNA allele, HEG , that contains the gene for the endonuclease. Thus, the gene is copied to the allele that initially did not have it and it is propagated through successive generations. This process
56-414: The perpetuation of the genetic elements that encode them independent of providing a functional attribute to the host organism. Homing endonuclease recognition sequences are long enough to occur randomly only with a very low probability (approximately once every 7 × 10 bp ), and are normally found in one or very few instances per genome . Generally, owing to the homing mechanism, the gene encoding
64-470: The several respects: Currently there are six known structural families. Their conserved structural motifs are: The yeast homing endonuclease PI-Sce is a LAGLIDADG-type endonuclease encoded as an intein that splices itself out of another protein ( P17255 ). The high-resolution structure reveals two domains : an endonucleolytic centre resembling the C-terminal domain of Hedgehog proteins , and
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