MAL6 of Saccharomyces: A complex genetic locus containing three genes required for maltose fermentation

The MAL6 locus is one of five closely related unlinked loci, any one of which is sufficient for fermentation of maltose in Saccharomyces. Previous genetic analysis indicated that this locus is defined by two complementation groups, MALp and MALg. MALp reportedly is a regulatory gene required for inducible synthesis of the two enzymatic functions needed for fermentation: maltose permease and maltase. We have investigated the physical and genetic structure of the MAL6 locus, which has been isolated on a recombinant DNA plasmid. One subclone of the region, pDF-1, was found to encode a single transcribed region and to contain the MALp gene. A second subclone, p1, was shown to contain the MALg function but surprisingly had not one but two maltose-inducible transcripts. Subclones having only one of these transcribed regions lacked MALg activity. The three transcribed regions have been named MAL61 and MAL62, which correspond to MALg, and MAL63, which corresponds to MALp. Thus clustered arrangement of a regulatory gene adjacent to the sequences it controls has not previously been described in eukaryotes and is reminiscent of bacterial operons except that the messenger RNA molecules are not polycistronic.