GENETIC EVIDENCE FOR A SILENT SUC GENE IN YEAST

Marian Carlson ¹, Barbara C. Osmond ¹, and David Botstein ¹

¹ Department of Biology, Massachusetts Institute of Technology, Cambridge, Mass. 02139

The SUC genes (SUC1–SUC7) of Saccharomyces are a family of genes that are dispersed in the yeast genome. A SUC⁺ allele at any locus confers the ability to produce the enzyme invertase and, thus, to ferment sucrose. Most yeast strains do not carry SUC⁺ alleles at all possible SUC loci. We have investigated the naturally occurring negative (suc⁰) alleles present at SUC loci with the aim of distinguishing between two possible models for the structure of suc⁰ alleles: (1) suc⁰ alleles correspond to a simple absence of SUC genetic information; (2) suc⁰ alleles are "silent" SUC genes that either produce a defective product or are not expressed. To facilitate these studies, sucrose-nonfermenting strains were constructed that are congenic to S. cerevisiae strain S288C (SUC2⁺), but carry at the SUC2 locus the naturally occurring negative allele, suc2⁰, of strain FL100 (Lacroute 1968). These strains were used to study the genetic properties of the suc2⁰ allele of FL100 and the suc⁰ alleles (suc1⁰, suc3⁰, etc.) of S288C. The suc2⁰ allele was shown to revert to an active Suc⁺ state and to provide functional information at three points in the SUC2 gene in recombination experiments; this suc2⁰ gene thus appears to be a "silent" gene. Similar tests for silent SUC genes in S288C (corresponding to loci other than SUC2) failed to reveal any additional silent genes.

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