Repressers, Cryptic Enzymes and Structural Genes

Abstract

The Multiple Alleles at the MZ Locus. The fermentation of melezitose¹ by Saccharomyces is controlled by a gene which generates a single enzyme capable of hydrolysing the five α-glucosides, namely, turanose, maltose, sucrose, α-methyl-glucoside and melezitose². The gene exists in a series of multiple alleles which differ from wild-type by the loss of capacity to react adaptively to one or more of the five α-glucosidic inductors^3,4. Only seven (of 32 theoretically possible) alleles are known: TMSGZ, TMSG-, TMS-Z, TMS- -, TM- - -, T- - - - and - - - - -. (Possibly only six real alleles of MZ exist; the putative TMS-Z allele may be a double mutant—TMSGZtogether with a gene generating a represser of G.) Each of the capital letters indicates ability to respond adaptively to the inductor indicated. Each of the mutant genotypes is stable on vegetative transfer and retains its characteristics without recombination in hybrids, thus establishing each mutant as a genuine allele⁵. The adaptive enzyme generated by MZ by exposure to each of the different substrates is identical irrespective of the allele or the inductor; the negatives fail to respond adaptively to the inductors indicated. Cell-free preparations and whole living cells, in which the enzyme has been induced, split all five substrates at characteristically similar rates⁶, indicating that permeability is not a factor in differentiating the different phenotypes; it has been inferred that the enzyme appears quickly after induction in the nucleoprotein layer⁷.