Repeated Family of Genes Controlling Maltose Fermentation inSaccharomyces carlsbergensis
Maltose fermentation inSaccharomycesspp. requires the presence of any one of five unlinked genes:MAL1, MAL2, MAL3, MAL4, orMAL6.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. TheMALgenes may code for one or more of these proteins. Recently a DNA fragment containing a maltase structural gene has been cloned from aMAL6strain, 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 crypticMALgenes in CB11,MAL1gandMAL3g(linked toMAL1and toMAL3, respectively), in addition to theMAL6locus. The physical analysis, which used a subclone of plasmid pMAL9-26 as a probe, detected threeHindIII genomic fragments with homology to the probe. Each fragment was shown to be linked to one of theMALloci genetically demonstrated to be present in CB11. Our results indicate that the cloned maltase structural gene in plasmid pMAL9-26 is linked toMAL6.Since theMAL6locus has previously been shown to contain a regulatory gene, theMAL6locus 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 theMAL6-derived probe shows that eitherMAL2andMAL4are not related toMAL6, or the DNA corresponding to these genes is absent from theMAL6strain CB11.