Development of papillae on colonies of two isopolyauxotrophic strains of Saccharomyces cerevisiae allelic in RAD6 during adenine starvation.

Papilla formation on colonies of two isopolyauxotrophic strains (ade2 his3 leu2 trp1 ura3) allelic in RAD6 was compared in order to find proper conditions for selecting mutants of Saccharomyces cerevisiae with altered starvation-induced mutability. The most promising for this purpose appeared to be culturing low numbers of colonies on suboptimal plates with a growth-limiting amount of adenine at 28 degrees C for 20 d. Inactivation of the RAD6 gene which suppresses the level of starvation-associated mutagenesis markedly enhanced papilla formation under these conditions. Formation of almost all papillae on 20-d-old colonies of BJC3 was caused by mutation. Most of the papillae (75%) were white Ade+ revertants. Three groups of these papillae were distinguished (Ade+, Ade+Rad6+ and Ade+Trp+). Both, Ade+Rad6+ and Ade+Trp+ double reversions were very probably caused by a suppressor mutation. The less frequent red papillae had the same auxotrophic markers and UV sensitivity as BJC3 but their outgrowth in liquid media was greater. It appears that creation of these papillae is caused by mutation affecting the cell response to growth limitation by low concentrations of adenine.