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
Hin
dIII 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.
Repeated Family of Genes Controlling Maltose Fermentation inSaccharomyces carlsbergensis