cis-Dominant mutations which dramatically enhance DUR1,2 gene expression without affecting its normal regulation.

We have isolated three cis-dominant mutations which dramatically enhance DUR1 ,2 gene expression in Saccharomyces cerevisiae. The mutant phenotype, which is expressed both in haploid and MATa/MAT alpha diploid strains, does not appear to be an alteration of the normal control system for this gene because its expression remained fully inducible and sensitive to nitrogen catabolite repression. Instead, we found much higher levels of DUR1 ,2-specific RNA under both uninduced and induced conditions, i.e., the overproduction trait was superimposed on normal regulation of the gene. The mutations seemed to affect gene expression in a unidirectional manner or to be specific for DUR1 ,2 gene expression, because other genes in proximity to the mutations were not affected. We feel that these mutations may alter the chromatin structure in the vicinity of the DUR1 ,2 upstream control sequences or, alternatively, may be Ty insertions which no longer possess the ROAM characteristics reported by others and ourselves.