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It was previously shown that MRG19 downregulates carbon metabolism in Saccharomyces cerevisiae upon glucose exhaustion, and that the gene is glucose repressed. Here, it is shown that glucose repression of MRG19 is overcome upon nitrogen withdrawal, suggesting that MRG19 is a regulator of carbon and nitrogen metabolism. [beta]-Galactosidase activity fostered by the promoter of GDH1/3, which encode anabolic enzymes of nitrogen metabolism, was altered in an MRG19 disruptant. As compared to the wild-type strain, the MRG19 disruptant showed a decrease in the ratio of 2-oxoglutarate to glutamate under nitrogen-limited conditions. MRG 19 disruptants showed reduced pseudohyphal formation and enhanced sporulation, a phenomenon that occurs under conditions of both nitrogen and carbon withdrawal. These studies revealed that MRG19 regulates carbon and nitrogen metabolism, as well as morphogenetic changes, suggesting that MRG19 is a component of the link between the metabolic status of the cell and the corresponding developmental pathway. Abbreviations: GOGAT, glutamate synthase; GS, glutamine synthetase; RTG, retrograde.

Studies on the Saccharomyces cerevisiae GAL/MEL genetic switch have revealed that its bistability is dependent on ultrasensitivity that can be altered or abolished by disabling different combinations of nested feedback loops. In contrast, we have previously demonstrated that weakening of the interaction between Gal80p and Gal4p alone is sufficient to abolish the ultrasensitivity (Das Adhikari et al. 2014). Here, we demonstrate that altering the epistatic interaction between Gal80p and Gal4p also abolishes the bistability, and the switch response to galactose becomes graded instead of binary. However, the GAL/MEL switch of wild-type and epistatically altered strains responded in a graded fashion to melibiose. The properties of the epistatically altered strain resemble Kluyveromyces lactis, which separated from the Saccharomyces lineage 100 mya before whole-genome duplication (WGD). Based on the results reported here, we propose that epistatic interactions played a crucial role in the evolution of the fine regulation of S. cerevisiae GAL/MEL switch following WGD. [ABSTRACT FROM AUTHOR]

In S . cerevisiae, following the Whole Genome Duplication ( 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 gal3Δ 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 ( UASG). We demonstrate that the GAL switch becomes independent of GAL3 by altering the interaction between Gal4p and Gal80p without altering the configuration of UASG. 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 ScGAL switch are similar to the features of the GAL switch of K . lactis that diverged from S . cerevisiae before the WGD. [ABSTRACT FROM AUTHOR]

We have reported previously that multiple copies of MRG19 suppress GAL genes in a wild-type but not in a gal80 strain of Saccharomycescerevisiae. In this report we show that disruption of MRG19 leads to a decrease in GAL induction when S. cerevisiae is induced with 0.02% but not with 2.0% galactose. Disruption of MRG19 in a gal3 background (this strain shows long-term adaptation phenotype) further delays the GAL induction, supporting the notion that its function is important only under low inducing signals. As a corollary, disruption of MRG19 in a gal80 strain did not decrease the constitutive expression of GAL genes. These results suggest that MRG19 has a role in GAL regulation only when the induction signal is weak. Unlike the effect on GAL gene expression, disruption of MRG19 leads to de-repression of CYCl-driven β-galactosidase activity. MRG19 disruptant also showed a twofold increase in the rate of oxygen uptake as compared with the wild-type strain. ADH2, CTA1, DLD1, and CYC7 promoters that are active during nonfermentative growth did not show any de-repression of β-galactosidase activity in the MRG19 disruptant. Western blot analysis indicated that MRG19 is a glucose repressible gene and is expressed in galactose and glycerol plus lactate. Experiments using green fluorescent protein fusion constructs indicate that Mrgl9p is localized in the nucleus consistent with the presence of a consensus nuclear localization signal sequence. Based on the above results, we propose that Mrgl9p is a regulator of galactose and nonfermentable carbon utilization. [ABSTRACT FROM AUTHOR]