AUTOREGULATION OF GAL4 TRANSCRIPTION IS ESSENTIAL FOR RAPID GROWTH OFKLUYVEROMYCES-LACTIS ON LACTOSE AND GALACTOSE

Transcriptional induction of genes in the lactose-galactose regulon of the yeast Kluyveromyces lactis requires the GAL4 transcription activa tor protein. Previous data indicated that the concentration of GAL4 wa s tightly regulated under basal, inducing, and glucose repressing cond itions but the mechanisms were unknown. In this paper we demonstrate t hat transcription of the GAL4 gene (Kl-GAL4) increases 3- to 4-fold du ring induction of the regulon. This increase requires a Kl-GAL4 bindin g site, UAS(G), in front of the Kl-GAL4 gene, indicating that the Kl-G AL4 protein autoregulates transcription of its own gene. Our data demo nstrate that the autoregulatory circuit is essential for full inductio n of the lactose-galactose regulon and, hence, for rapid growth on lac tose or galactose. Other data indicate that basal transcription of the Kl-GAL4 gene is governed by unidentified promoter elements. The exist ence of the autoregulatory circuit reveals an important difference bet ween the lactose-galactose regulon and its homologue in Saccharomyces cerevisiae, the melibiose-galactose regulon. This difference may have evolved in response to different selective pressures encountered by th e two organisms.