Perturbation of the interaction between Gal4p and Gal80p of the S accharomyces cerevisiae GAL switch results in altered responses to galactose and glucose

Summary In S . cerevisiae , following the W hole G enome D uplication ( WGD ), GAL1 ‐encoded galactokinase retained its signal transduction function but lost basal expression. On the other hand, its paralogue GAL3 , lost kinase activity but retained its signalling function and basal expression, thus making it indispensable for the rapid induction of the S . cerevisiae   GAL switch. However, a gal3 Δ strain exhibits delayed growth kinetics due to the redundant signalling function of GAL1 . The subfunctionalization between the paralogues GAL1 and GAL3 is due to expression divergence and is proposed to be due to the alteration in the Upstream Activating Sequences ( UAS G ). We demonstrate that the GAL switch becomes independent of GAL3 by altering the interaction between Gal4p and Gal80p without altering the configuration of UAS G . In addition to the above, the altered switch of S . cerevisiae loses ultrasensitivity and stringent glucose repression. These changes caused an increase in fitness in the disaccharide melibiose at the expense of a decrease in fitness in galactose. The above altered features of the Sc GAL switch are similar to the features of the GAL switch of K . lactis that diverged from S . cerevisiae before the WGD .