RECOVERY OF GENE-FUNCTION BY GENE DUPLICATION IN SACCHAROMYCES-CEREVISIAE

A prototroph revertant (Rev9) selected from an ATCase(-) mutant of the URA2 gene containing three nonsense mutations was shown to contain tw o ATCase coding sequences. We cloned both ATCase coding areas to show that the duplicated locus (dl9) was the only functional one. Its size corresponded roughly to the second half of the URAL, wild-type gene. S equence analysis of the 5' end of dl9 indicated that this duplicated s equence was inserted within the intergenic region close to the MRS3 ge ne and was transcribed from an unknown promoter divergently from the M RS3 gene. The event leading to the revertant strain Rev9 included a re arrangement that increased the size of chromosome X by about 60 kb. In agreement with such a rearrangement, recombination was undetectable i n the vicinity of the locus dl9. Genetic mapping confirms that the MRS 3 gene is 2 cM distal to the URA2 gene on the right arm of chromosome X.