Abstract
Thirty-two mutants with improved growth response on a yeast-sucrose compared with a defined medium have been characterized with respect to ribonucleoside supplementability. Twenty mutants respond to either pyrimidine ribonucleoside. Four mutants respond to one or both purine ribonucleosides. Eight mutants ("putative" auxotrophs) do not respond to dietary RNA supplementation. Mapping and complementation studies suggest that eleven loci are represented: one, rudimentary, probably accounts for all pyrimidine requirers; there are three purine loci and seven at which the putative auxotrophs are found.