FEMS Microbiology Letters 77 (1991) 233-236 © 1991 Federation of European Microbiological Societies 0378-1097/91/$03.50 ADONIS 037810979100090B FEMSLE 04269 233 Isolation and characterization of maltose non utilizing (mnu) mutants mapping outside the MALl locus in Saccharomyces cerevisiae * Marco Vanoni 1 and Michael J. Goldenthal 2 1 Dipartimento Fisiologia e Biochimica Generali, Universitgt degli Studi di Milano, Milan, Italy, and 2 Rutgers University, Camden, N J, U.S.A. Department of Biology, Received 3 August 1990 Revision received 7 September 1990 Accepted 9 September 1990 Key words: Maltase; Mutant isolation; MAL Genes; Yeast; Sugar 1. SUMMARY The MALl locus of Saccharomyces cerevisiae comprises three genes necessary for maltose utili- zation. They include regulatory, maltose transport and maltase genes designated MAL1R, MAL1T and MALLS respectively. Using a MALl strain transformed with an episomal, multicopy plasmid carrying the MAL2 locus, five recessive and one dominant mutant unable to grow on maltose, but still retaining a functional MALl locus were iso- lated. All the mutants could use glycerol, ethanol, raffinose and sucrose as a sole carbon source; expression of the maltase and maltose permease genes was severely and coordinately reduced. Only the dominant mutant failed to accumulate the MAL1R mRNA. Correspondence to: M. Vanoni, Dipartimento Fisiologia e Bio- chimica Generali, Universith degli Studi di Milano, via Celoria 26, 1-20133 Milan, Italy. . A preliminary account of this work has been presented at the meeting: Gene Expression and Regulation: the Legacy of Luigi Gorini, held in Milan, Italy on 12-14 October 1987. 2. INTRODUCTION The genetic loci (MAL1-MAL4, MAL6) that allow Saccharomyces yeasts to utilize maltose as a sole carbon source have been studied since the early fifties (ref. 1, reviewed in refs. 2 and 3). Genetic and physical analysis have shown that all yeast strains, either Mal + or Mal-, carry MAL-re- lated sequences at the MALl locus, in either a functional or non-functional form [4,5]. Each MAL locus is composed of at least three closely linked genes: MALS, the structural gene for maltase [6]; MALT, encoding maltose permease or one of its subunits [7]: MALR [8] whose product coordi- nately regulates transcription of the MALS and MALT genes [9,10]. Expression of the MAL genes is not only in- duced by maltose but is also affected by carbon catabolite repression [11,12] a global regulatory system that affects expression of sugar fermenta- tion genes, as well as of genes encoding enzymes of the tricarboxylic acid cycle, the glyoxylate shunt and gluconeogenesis [11-13]. Other genes, in ad- dition to the MAL genes themselves, have been reported to affect maltose utilization (ref. 3 and