[Cryptic heterokaryons in Saccharomyces cerevisiae strains: pseudopleiotropic suppression in the multiple marked...

The yeast strain YPH857 carrying multiple genetic markers was shown to segregate clones that had the pleiotropic suppression phenotype. The phenotype was designated Ppsu+. This suppression involves deletion alleles of the TRP1 and HIS3 genes and an insertion in the URA3 gene. Unlike the original YPH857 culture that carries an unidentified mutation of resistance to cycloheximide, the Ppsu+ clones exhibited a decreased level of resistance to this inhibitor of protein synthesis. In addition, they have a lower mating ability and can produce asci on a standard medium for sporulation. A comparative analysis of total DNA from the YPH857 strain and Ppsu+ segregants by Southern blotting provided evidence for the presence of an extraneous nucleus in these segregants. Ppsu+ strains were shown to contain wild-type alleles, apart from deletion and insertional alleles typical for the YPH857 strain. Moreover, they contain the 2 microns DNA of the Scp3 type with deletion of one of the two EcoRI and HpaI recognition sites (whereas the 2 microns DNA of YPH857 belongs to the Scp1 type) and exhibit heterogeneity with respect to the presence of one EcoR1 recognition site in the gene of 5S ribosomal RNA. It was supposed that the "cryptic" nucleus belongs to a strain of low viability and can survive as an unexpressed DNA in a small fraction of cells. Nuclei from the cryptic and YPH857 strains can be fused at a low rate to yield Ppsu+ cells capable of sporulation. In certain cases, a Ppsu+ clone may be heterogeneous: some of its cells contain nuclei in an unfused heterokaryotic state. This assumption has been confirmed by selecting Cyhr colonies with the original YPH857 genome among Ppsu+ clones on the cycloheximide-containing medium.