Trehalose and maltose metabolism in yeast transformed by a MAL4 regulatory gene cloned from a constitutive donor strain

Accumulation of trehalose in Saccharomyces cerevisiae growing on maltose is dependent on the TPS1 gene encoding the UDPglucose-linked trehalose synthase

When yeast strains were cultivated on maltose, the synthesis of trehalose already started in the exponential phase of growth, well before exhaustion of the sugar from the medium. This active pattern of trehalose accumulation was also observed in a maltose constitutive mutant strain growing on glucose. However, this accumulation was completely prevented by deletion of the TPS1 gene coding for the catalytic subunit of the UDPglucose-linked trehalose-6-phosphate synthase, indicating that no alternative pathway for trehalose synthesis exists in yeast. The active pattern of trehalose accumulation seems to be consistent with the finding that trehalose-6-phosphate synthase is more active in strains growing on maltose than on glucose.

Cloning and characterization of baker's yeast alpha-glucosidase: over-expression in a yeast strain devoid of vacuolar proteinases

Two alpha-glucosidase (maltase) genes, designated GLUCPI and GLUCPII, have been cloned from an industrial strain of baker's yeast (Saccharomyces cerevisiae) by complementation of a maltase-negative mutant strain. The different genes were identified according to their alternatively expressed isoenzymes PI and PII in transformants after isoelectric focusing and activity staining in separated cell lysates. The gene encoding alpha-glucosidase PI (GLUCPI), which was not present in laboratory strains of S. carlsbergensis with a defined MAL1, 2, 3, 4 or 6 locus, was sequenced and compared with the recently published MAL6S gene. This comparison revealed single amino acid deviations at three positions in the predicted polypeptide sequence. In addition, the divergent promoter region of GLUCPI differed from MAL6S by a triple repeated 147-bp DNA segment. Maltose induction and glucose repression of alpha-glucosidase PI were not affected by the deletion of the repeated DNA segment. However, the absolute expression of alpha-glucosidase PI increased two- to four-fold. In addition, a two-fold increase in the maltase synthesis occurred when the cloned positive regulator gene MAL2-8ep was on the same plasmid. Furthermore, stability of the alpha-glucosidase in cultures in the stationary growth phase was greatly enhanced using a host strain lacking the proteinases A and B and the carboxypeptidases Y and S. Promoter trimming, MAL2-8cp stimulation and the use of a host strain deficient in four vacuolar proteinases resulted in alpha-glucosidase PI expression of about 13% of the soluble protein.

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.