Identification of upstream activator sequences that regulate induction of the beta-galactosidase gene in Kluyveromyces lactis.

Transcription of the Kluyveromyces lactis beta-galactosidase gene, LAC4, is inducible by galactose and lactose. We examined the effects of deletion mutations within the LAC4 promoter on the expression of beta-galactosidase activity. The results of these experiments indicate that at least two upstream activator sequences (UAS) mediate maximum induction by galactose. These UAS sequence elements are homologous to UAS that regulate induction of the melibiose-galactose regulon of Saccharomyces cerevisiae. We also show that a synthetic copy of one of the K. lactis UAS restores the inducibility of a deleted, noninducible LAC4 promoter. Since the uninduced or basal level of LAC4 expression was increased in several promoter deletion strains and in deletion strains carrying one or two synthetic UAS, we examined the contribution of the LAC9 positive regulatory protein to this effect. The LAC9 protein is thought to bind to UAS and activate transcription of LAC4 (L.V. Wray, M.M. Witte, R.C. Dickson, and M.I. Riley, Mol. Cell. Biol. 7:1111-1121, 1987). Our results demonstrate that LAC9 protein plays a role in setting the uninduced level of gene expression, but other factors also participate. For example, in a lac9 background a LAC4 promoter deletion mutant with two copies of a synthetic 17-base-pair UAS yields a sevenfold higher level of uninduced LAC4 expression than the same strain with one UAS. These and other data indicate that the basal level of gene expression is strongly influenced by the base sequence of the promoter.