Revertants of a Transcription Termination Mutant of Yeast Contain Diverse Genetic Alterations

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Revertants of the cyc1-512 transcription termination mutant of the yeast Saccharomyces cerevisiae were isolated and subjected to a detailed genetic analysis. The cyc1-512 mutation previously was shown to be a 38-base pair deletion that causes only 10% of the normal steady-state levels of CYC1 mRNA and of the CYC1 gene product, iso-1-cytochrome c. Forty-one cyc1-512 revertants were classified by their content of iso-1-cytochrome c and by their genetic properties in meiotic crosses. Many of the revertants contain local genetic changes that either partially or completely restore the level of iso-1-cytochrome c. One revertant was shown to contain an unlinked extragenic suppressor, designated sut1, that causes partial suppression of the transcription termination defect. Four revertants of cyc1-512 contain chromosomal rearrangements with breakpoints that are tightly linked to the CYC1 locus; these include one duplication, one possible inversion, and two reciprocal translocations. Detailed genetic mapping demonstrated that one of the reciprocal translocations is between the right arms of chromosomes X and XII, with a breakpoint mapping 3' to the CYC1 locus. These results indicate that the defect in transcription termination in cyc1-512 can be restored in a variety of ways, including the translocation of different chromosomal regions to the 3' end of the CYC1 locus, local changes presumably at or near the original defect, and by mutation at another locus distinct from CYC1.

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