STRUCTURE OF THE MULTIGENE FAMILY OF MAL LOCI IN SACCHAROMYCES SPECIES (MALTASE)

Abstract

Multigene families (MGF) are a ubiquitous feature of eukaryotes, however, in Saccharomyces, the presence of MGF is more limited. Early studies showed that there were five unlinked but functionally identical MAL loci (MAL1 through MAL4 and MAL6) that behaved as regulatory genes in governing the catabolism of maltose in yeast. These studies were unable to reveal any MAL structural genes. Later, each MAL locus was proposed to be a complex of a MAL regulatory gene and MAL structural gene(s); only the regulatory determinant could be identified because it is present less frequently than the MAL structural determinant(s).;Since the MAL structural determinants could not be revealed by conventional genetic means, a cloned maltase gene from the MAL6 locus was used to analyze the structural genes of other MAL loci. These results indicated that all the MAL loci are closely related and that MAL1 is possibly the ancestral locus from which the other MAL loci were generated by transposition. Subcloning the MAL6 locus, the use in complementation studies and insertional mutagenesis of the MAL1 locus showed that the two loci are highly homologous to one another. These DNA sequences could be characterized as comprising a cluster of genes (within 7 kbp), carrying MALS, MALT and MALR, representing the maltase, maltose permease and regulatory genes, respectively. Further analyses of cloned DNA showed that all the known MAL loci share a homologous 9-10 kbp region that includes MALS, MALT, MALR and regions 2-3 kbp downstream of the 3' end of MALS. mal1(DEGREES)-12 and mal1(DEGREES)-15 (two partially active alleles of MAL1) show a deletion and a divergence of their malR sequences, respectively. MAL3 and mal1(DEGREES)-15 DNA showed an abrupt divergence in the 3' end of their MALS sequences due to the presence of the 5' end of invertase gene (SUC) sequences. MAL3 and MAL6 showed a substantial duplication of their MALR sequences, with MAL3 carrying a 10 kbp direct repeat and MAL6 carrying a 2.5 kbp direct repeat. The significance of these duplications is discussed.