Identification of a second trans-acting gene controlling maltose fermentation in Saccharomyces carlsbergensis.

Maltose fermentation in Saccharomyces spp. requires the presence of a dominant MAL locus. The MAL6 locus has been cloned and shown to encode the structural genes for maltose permease (MAL61), maltase (MAL62), and a positively acting regulatory gene (MAL63). Induction of the MAL61 and MAL62 gene products requires the presence of maltose and the MAL63 gene. Mutations within the MAL63 gene produce nonfermenting strains unable to induce the two structural gene products. Reversion of these mal63 nonfermenters to maltose fermenters nearly always leads to the constitutive expression of maltase and maltose permease, and constitutivity is always linked to MAL6. We demonstrated that for one such revertant, strain C2, constitutivity did not require the MAL63 gene, since deletion disruption of this gene did not affect the constitutive expression of the structural genes. In addition, constitutivity was trans acting. Deletion disruption of the MAL6-linked structural genes for maltase and maltose permease in this strain did not affect the constitutive expression of a second, unlinked maltase structural gene. We isolated new maltose-fermenting revertants of a nonfermenting strain which carried a deletion disruption of the MAL63 gene. All 16 revertants isolated expressed maltase constitutively. In one revertant studied in detail, strain R10, constitutive expression was demonstrated to be linked to MAL6, semidominant, trans acting, and residing outside the MAL63-MAL61-MAL62 genes. From these studies we propose the existence of a second trans-acting regulatory gene at the MAL6 locus. We call this new gene MAL64. We mapped the MAL64 gene 2.3 centimorgans to the left of MAL63. The role of the MAL64 gene product in maltose fermentation is discussed.     

Mutational analysis of the MAL1 locus of Saccharomyces: identification and functional characterization of three genes

Summary Fermentation of maltose by Saccharomyces strains depends on the presence of any one of five unlinked MAL loci (MAL1, MAL2, MAL3, MAL4 or MAL6). Earlier mutational analyses of MAL2 and MAL6 containing strains have identified a single complementation group at each of these two loci. However complementation analysis between naturally occurring Mal^– Saccharomyces strains isolated from the wild demonstrated the presence of two complementation groups (designated MALp and MALg) at the MAL1, MAL3 and MAL6 loci. The available evidence suggests that the MALp gene is functionally equivalent to the complementation group identified by mutational analysis at the MAL6 locus and that this gene encodes a protein involved in the regulation of the coordinate induction of both maltase and maltose permease synthesis.In this paper we report the isolation, in a well characterized MAL1 strain, of 47 mutants unable to ferment maltose. All the mutants, with one exception, map at the MAL1 locus. These mal1 mutants, except for one, are recessive to MAL1 and fall into two major complementation groups. Evidence is presented that these two classes of mutants identify both a gene involved in the regulation of maltose fermentation (MAL1R) and a gene involved in maltose transport (MAL1T). We also report here the isolation of a temperature sensitive maltose nonfermenting mutant mapping at the MAL1 locus identifying a third gene (MAL1S) at this locus. The maltase synthesized by this mutant, when assayed in cell-free extracts, is significantly more thermolabile than the wild type enzyme. Our findings demonstrate that MAL1 is a complex locus comprising at least three genes: MAL1R, a gene involved in the coordinate regulation of the synthesis of maltase and maltose transport; MAL1T, a gene encoding a component of the maltose transport system; and MAL1S, a likely candidate for the structural gene for maltase.     

The Constitutive, Glucose-Repression-Insensitive Mutation of the Yeast MAL4 Locus Is an Alteration of the MAL43 Gene

Mutations resulting in constitutive production of maltase have been identified at each of the five MAL loci of Saccharomyces yeasts. Here we examine a dominant constitutive, glucose-repression-insensitive allele of the MAL4 locus (MAL4-C). Our results demonstrate that MAL4-C is an alteration in the MAL43 gene, which encodes the positive regulator of the MAL structural genes, and that its product is trans-acting. The MAL43 gene from the MAL4-C strain was cloned and integrated into a series of nonfermenting strains lacking a functional regulatory gene but carrying copies of the maltose permease and maltase structural genes. Expression of the maltase structural gene was both constitutive and insensitive to glucose repression in these transformants. The MAL4-C allele also results in constitutive expression of the unlinked MAL12 gene (encoding maltase) in this strain. In addition, the cloned MAL43 gene was shown to be dominant to the wild-type MAL63 gene. We also show that most of the glucose repression insensitivity of strains carrying the MAL4-C allele results from alteration of MAL43.     

Identification of new genes involved in disaccharide fermentation in yeast

Summary Maltose non-fermenting mutants were obtained from strains carrying a MAL4 allele which permits constitutive synthesis of maltase. Cells carrying this allele are able to utilize sucrose in the absence of the classical sucrose genes. All maltose non-fermenting mutants were also sucrose non-fermenters. Eight mutants had become maltase negative; 19 mutants could still form maltase constitutively.In crosses with segregational maltose and sucrose non-fermenting strains, enzyme negative mutants gave diploids unable to ferment maltose and sucrose. Enzyme positive, non-fermenting mutants gave diploids which readily fermented maltose and sucrose. This latter type of mutants was designated dsf (disaccharide fermentation) mutants.The diploids derived from crossing non-fermenting mutants with segregational non-fermenters were subjected to tetrad analysis. Enzyme negative non-fermenters gave only non-fermenting progeny. The dsf mutants segregated both fermenting and non-fermenting progeny, some of which showed the dsf phenotype. This indicated that none of the dsf mutants had a defect in a gene closely linked to MAL4. Crosses between dsf mutants and strains carrying the maltose genes MAL2 and MAL3 showed that the mutations affected maltose fermentation in general. Sucrose fermentation in the presence of the classical sucrose gene SUC3 was not affected, nor were fermentation of glucose, fructose and galactose.The uptake of radioactivity from uniformly labeled maltose appeared to be blocked in mutants of at least four of the dsf genes. Only one non-leaky and a leaky mutant showed a significant uptake.These results suggest that there is an extremely complex transport system for maltose and sucrose or that the utilization of these disaccharides requires a complex series of metabolic reactions.     

Isolation and characterization of maltose non utilizing (mnu) mutants mapping outside the MAL1 locus in Saccharomyces cerevisiae

The MAL1 locus of Saccharomyces cerevisiae comprises three genes necessary for maltose utilization. They include regulatory, maltose transport and maltase genes designated MAL1R, MAL1T and MAL1S respectively. Using a MAL1 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 MAL1 locus were isolated. 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.     

Genetic control of maltase formation in yeast. III. Isolation and characterization of temperature-sensitive mutants affecting maltase induction and maltose utilization

Temperature-sensitive mutants affecting maltose utilization in the yeast Saccharomyces cerevisiae have been isolated. Two such mutants although failing to ferment maltose at the restrictive temperature, have normal induced level of maltase. The third mutant (UNT-37) not only failed to ferment maltose but has 5-6 fold less induced level of maltase at the restrictive temperature than the parental strain. The genetic control mechanisms of maltase induction and maltose utilization have been discussed.     

The Naturally Occurring Alleles of Mal1 in Saccharomyces Species Evolved by Various Mutagenic Processes Including Chromosomal Rearrangement

In order for a yeast strain to ferment maltose it must contain any one of the five dominant MAL loci. Each dominant MAL locus thus far analyzed contains three genes: GENE 1, encoding maltose permease, GENE 2 encoding maltase and GENE 3 encoding a positive trans-acting regulatory protein. In addition to these dominant MAL loci, several naturally occurring, partially functional alleles of MAL1 and MAL3 have been identified. Here, we present genetic and molecular analysis of the three partially functional alleles of MAL1: the MAL1p allele which can express only the MAL activator; the MAL1 g allele which can express both a maltose permease and maltase; and the mal1(0) allele which can express only maltase. Based on our results, we propose that the MAL1p, MAL1g and mal1(0) alleles evolved from the dominant MAL1 locus by a series of rearrangements and/or deletions of this yeast telomere-associated locus as well as by other mutagenic processes of gene inactivation. One surprising finding is that the MAL1g-encoded maltose permease exhibits little sequence homology to the MAL1-encoded maltose permease though they appear to be functionally homologous.     

Distribution of mannosamine and mannosaminuronic acid among cell walls of Bacillus species

Some Escherichia coli K-12 lamB mutants, those producing reduced amounts of LamB protein (one-tenth the wild type amount), grow normally on dextrins but transport maltose when present at a concentration of 1 microM at about one-tenth the normal rate. lamB Dex- mutants were found as derivatives of these strains. These Dex- mutants are considerably impaired in the transport of maltose at low concentrations (below 10 microM), and they have a structurally altered LamB protein which is impaired in its interaction with phages lambda and K10 but still interacts with a lambda host range mutant lambda hh*. The Dex- mutants are double lamB mutants carrying one mutation, already present in the parental strains, that reduces LamB synthesis and a second that alters LamB structure. The secondary mutations, present in different independent Dex- mutants, are clustered in the same region of the lamB gene. Dex+ revertants were isolated and analyzed: when the altered LamB protein is made in wild-type amount, due to a reversion of the first mutation, the phenotype reverts to Dex+. However, these Dex+ revertants are still very significantly impaired in maltose transport at low concentrations (below 10 microM).     

The Effects of Three Different MAL Loci on the Regulation of Maltase Synthesis in Yeast

Inbred haploid strains of Saccharomyces cerevisiae carrying MAL1, MAL2 or MAL6 in a common background have been crossed to each other and to strains carrying no active MAL loci. The kinetics of maltase induction and the induced maltase levels have been examined in the inbred strains and in haploid segregants of the crosses. Differences have been found in the kinetics of induction and induced maltase levels that segregate with the different MAL loci. In the strains tested, the relative rates of maltase induction were MAL2>MAL6>>MAL1; the relative induced maltase levels were MAL2>MAL6~MAL1. These results indicate that MAL1, MAL2 and MAL6 are (or include) regulatory genes that control the accumulation of the enzymes of maltose fermentation.     

Detection of maltose fermentation genses in the baking yeast strains of Saccharomyces cerevisiae

Y.ODA AND K. TONOMURA. 1996. The presence of any one of the five unlinked MAL loci ( MALI, MAL2, MAL3, MAL4 AND MAL6 ) confers the ability fo ferment maltose on the yeast Saccharomyces cerecvisiae . Each locus is composed of three genes encoding maltose permease, α-glucosididase and MAL activator. Chromosomal DNA of seven representative baking strains has been separated by pulse-field gel electrophoresis and probed with three gense in MAL6 locus. The DNA bands to which all of the three MAL derived probes simultaneously hybridized were chromosome VII carrying MAL1 in all of the strains tested, chromosome XI carrying MAL4 in six strains, chromosome III carrying mal2 in three strains and chromosomes II and VIII carrying MAL3 and MAL6 , respectively, in the one strain. The number of MAL loci in bakig strains was comparable to those of brewing strins.     

Identification and characterization of the maltose permease in genetically defined Saccharomyces strain

Saccharomyces yeasts ferment several alpha-glucosides including maltose, maltotriose, turanose, alpha-methylglucoside, and melezitose. In the utilization of these sugars transport is the rate-limiting step. Several groups of investigators have described the characteristics of the maltose permease (D. E. Kroon and V. V. Koningsberger, Biochim. Biophys. Acta 204:590-609, 1970; R. Serrano, Eur. J. Biochem. 80:97-102, 1977). However, Saccharomyces contains multiple alpha-glucoside transport systems, and these studies have never been performed on a genetically defined strain shown to have only a single permease gene. In this study we isolated maltose-negative mutants in a MAL6 strain and, using a high-resolution mapping technique, we showed that one class of these mutants, the group A mutants, mapped to the MAL61 gene (a member of the MAL6 gene complex). An insertion into the N-terminal-coding region of MAL61 resulted in the constitutive production of MAL61 mRNA and rendered the maltose permease similarly constitutive. Transformation by high-copy-number plasmids containing the MAL61 gene also led to an increase in the maltose permease. A deletion-disruption of MAL61 completely abolished maltose transport activity. Taken together, these results prove that this strain has only a single maltose permease and that this permease is the product of the MAL61 gene. This permease is able to transport maltose and turanose but cannot transport maltotriose, alpha-methylglucoside, or melezitose. The construction of strains with only a single permease will allow us to identify other maltose-inducible transport systems by simple genetic tests and should lead to the identification and characterization of the multiple genes and gene products involved in alpha-glucoside transport in Saccharomyces yeasts.     

Genetic control of maltase formation in yeast

Summary In an attempt to resolve the question of structural versus regulatory genes, we have isolated several maltase negative mutants from strain 1403-7A, which carries the MAL4 gene. Antibody required for 50% inhibition of enzyme activity in these mutant strains is directly proportional to the amount of activity present, and no evidence was found for the presence of immunologically cross reacting material. These results suggest that either a gene closely linked to the MAL4 gene has a regulatory function or the MAL4 gene itself is a regulatory gene.     

Gene dosage effects on the synthesis of maltase in yeast.

Inbred strains of Saccharomyces cerevisiae carrying MAL1, MAL2, or MAL6 in a common background were used to construct (i) homo- or heterozygous diploids carrying one or two active alleles of a single MAL locus (MAL1, MAL2, or MAL6) and (ii) triploids carrying one, two, or three active alleles of MAL2. The diploid and triploid strains were used to investigate gene dosage effects of the differential rate of maltase synthesis (delta enzyme activity/delta growth) and the kinetics of induction (for MAL2). All three MAL loci exhibited a gene dosage effect on the differential rate of maltase synthesis; MAL2 also exhibited a gene dosage effect on the kinetics of induction. The dosage effects of the MAL1 and MAL6 loci were additive, but the effects of the MAL2 locus were not; the magnitude of the MAL2 gene dosage effect decreased with increasing dosage. These results are compatible with the current genetic evidence that the MAL genes are regulatory loci if the product(s) of the MAL1 and MAL6 locus is produced in limiting amounts but the product(s) of the MAL2 locus is produced in excess, except at very low genes dosages.     

Repeated Family of Genes Controlling Maltose Fermentation in Saccharomyces carlsbergensis

Maltose fermentation in Saccharomyces spp. requires the presence of any one of five unlinked genes: MAL1, MAL2, MAL3, MAL4, or MAL6. Although the genes are functionally equivalent, their natures and relationships to each other are not known. At least three proteins are necessary for maltose fermentation: maltase, maltose permease, and a regulatory protein. The MAL genes may code for one or more of these proteins. Recently a DNA fragment containing a maltase structural gene has been cloned from a MAL6 strain, CB11, to produce plasmid pMAL9-26. We have conducted genetic and physical analyses of strain CB11. The genetic analysis has demonstrated the presence of two cryptic MAL genes in CB11, MAL1g and MAL3g (linked to MAL1 and to MAL3, respectively), in addition to the MAL6 locus. The physical analysis, which used a subclone of plasmid pMAL9-26 as a probe, detected three HindIII genomic fragments with homology to the probe. Each fragment was shown to be linked to one of the MAL loci genetically demonstrated to be present in CB11. Our results indicate that the cloned maltase structural gene in plasmid pMAL9-26 is linked to MAL6. Since the MAL6 locus has previously been shown to contain a regulatory gene, the MAL6 locus must be a complex locus containing at least two of the factors needed for maltose fermentation: the structural gene for maltase and the maltase regulatory protein. The absence of other fragments which hybridize to the MAL6-derived probe shows that either MAL2 and MAL4 are not related to MAL6, or the DNA corresponding to these genes is absent from the MAL6 strain CB11.     

A Suppressor of snf1 Mutations Causes Constitutive High-Level Invertase Synthesis in Yeast

The SNF1 gene product of Saccharomyces cerevisiae is required to derepress expression of many glucose-repressible genes, including the SUC2 structural gene for invertase. Strains carrying a recessive snf1 mutation are unable to ferment sucrose. We have isolated 30 partial phenotypic revertants of a snf1 mutant that were able to ferment sucrose. Genetic characterization of these revertants showed that the suppressor mutations were all recessive and defined eight complementation groups, designated ssn1 through ssn8 (suppressor of snf1 ). The revertants were assayed for secreted invertase activity, and although activity was detected in members of each complementation group, only the ssn6 strains contained wild-type levels. Synthesis of secreted invertase in ssn6 strains was found to be constitutive, that is, insensitive to glucose repression; moreover, the ssn6 mutations also conferred constitutivity in a wild-type ( SNF1 ) genetic background and are, therefore, not merely suppressors of snf1 . Pleiotropic defects were observed in ssn6 mutants. Genetic analysis suggested that the ssn6 mutations are allelic to the cyc8 mutation isolated by R. J. Rothstein and F. Sherman, which causes increased production of iso-2-cytochrome c. The data suggest a regulatory function for SSN6 .