[Selective systems for obtaining recessive ribosomal suppressors in saccharomycete yeasts].

Recessive mutations only occurring in two genes (ribosomal suppressors sup1 and sup2) can be obtained using special selective system. We demonstrate that the absolute selectivity of the system is based on selection for simultaneous reversions to prototrophy in mutants requiring adenine and histidine in haploids marked by two different nonsense mutations--his7-1 (UAA) and ade1-14 (UGA, this being identified in the present study). In support to this conclusion, we developed an analogous system utilising his7-1 (UAA) and lys2-87 (UGA). The selectivity of the system is shown to be influenced both by the choice of nonsense alleles and by genotypic background.