Saccharomyces_A

Next the doublestranded break is repaired through a

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Unformatted text preview: some III at a specific 18 bp recognition sequence just to the right of a segment known as the Y segment in MAT. Next, the doublestranded break is repaired through a recombination-like process in which homologous pairing occurs between sequences from one of the storage loci, such as HML, and identical sequences surrounding MAT (Fig. A.11b). In this example, HML DNA then serves as a template to resynthesize DNA at MAT. The result is the replacement of MAT DNA with DNA from the storage locus. Overall the process is conservative, with the duplication of storagelocus DNA balanced by an equal loss of DNA from MAT. The information in the storage locus remains unchanged. The storage loci are silent in two respects. They are not transcribed, and even though they contain the 18 bp HOprotein-recognition sequence, they are not susceptible to double-stranded cutting by the HO enzyme. Both these properties result from the presence of “silencer” sequences at the boundaries of the storage loci, which are not present at the boundaries of MAT. In a process that may resemble the silencing of genes in higher eukaryotes, the silencer sequences in...
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