THE GENETIC INTEGRITY OF UPGRADED AND DOWNGRADED ALLELES OF THE MELEZITOSE LOCUS IN SACCHAROMYCES.

Abstract

The fermentation of melezitose by Saccharomyces is controlled by a single gene (MZ), which generates the adaptive enzyme, melezitase, capable of hydrolyzing the six alpha-glucosides, turanose, maltose, sucrose, palatinose, alpha-methyl-glucoside and melezitose. The induction of melezitase occurs when cells carrying MZ are exposed to specific alpha-glucosides (Lindegren and Lindegren, 1953; Palleroni and Lindegren, 1953; Hwang and Lindegren, in ms.). The allele responds to six alpha-glucosides and has been designated AIIZT'Oz (abbreviated to TMSGZ). A series of multiple alleles which differ from one another by their ability to react adaptively to the five specified alpha-glucosidic inductors have been identified and designated TAISGZ, TMSGz, TMSgz, TMsgz, Tmsgz and tmsgz. (The small letters indicate the inabilities of a culture to respond adaptively to the corresponding substrates.) Only six alleles are ltnown, where 32 would be expected, proving that the different phenotypes are the manifestations of a series of multiple alleles of one gene rather than permutation of five different nonallelic genes. The totipotent allele (TAISGZ) can be downgraded by ultraviolet irradiation to the partial alleles (ThlISGz, TMSgz, TMsgz and Tmsgz) and to the totally recessive allele (tmsgz). Evidence, previously presented, was inferred to show that the partial alleles and the totally recessive allele are upgraded by directed mutation to the totipotent allele following exposure to one of the alpha-glucosides, specifically nonutilizable by the particular mutant (e.g. melezitose) (Lindegren, Pittman and Ranganathan, 1957; Lindegren and Pittman, 1958; Lindegren, 1963a). T h e restoration to inducibility is an allor-none, single, controlled, mutational event (Lindegren and Pittman, 1959). The present communication confirms previous inferences (Lindegren, Pittman and Ranganathan, 1957) concerning the genetical integrity of the UV induced multiple alleles of the MZ locus. In the earlier report, the stability of the partial mutants was inferred f r o ~ n the analysis of 9 tetrads from hybrids between the downgraded mutants, TMSGz, TMSgz and Tmsgz and totipotent TMSGZ cultures. In the present study the genetical analysis, involving two generations (33 tetrads) of hybrids between the downgraded partial mutant TAIIsgz and a tmsgz culture, established thc integrity of the partial mutant T M s p In the previous work, the outcrosses were to the totipotent TMSGZ; in the present work, they wcre to the totally recessive allcle, tmsgz. The integrity of the upgraded allele, TMSGZ (not tested previously) was maintained in hybrids ~v i th tmsgz cultures over three generations (74 tetrads).