Contribution of Cat8 and Sip4 to the transcriptional activation of yeast gluconeogenic genes by carbon source-responsive elements

The carbon source-responsive element (CSRE) functions as an activating prom oter motif of gluconeogenic genes in Saccharomyces cerevisiae. The positive ly acting regulatory genes CATS and SIP4 encode CSRE-binding proteins which contribute unequally to the regulated expression of a CSRE-dependent repor ter gene (85% and 15%, respectively, under conditions of glucose derepressi on). Deregulated variants of Cat8 and Sip4 are able to bind to the CSRE and allow glucose-insensitive gene activation, even in the absence of the othe r protein, arguing against the physiological significance of heterodimer fo rmation. Gel retardation assays provide evidence for a different binding af finity of Cat8 and Sip4 to at least some CSRE sequence variants. Both effic ient biosynthesis of and transcriptional activation by Sip4 require a funct ional CAT8 gene, while Cat8 was not dependent on SIP4. Thus, our data sugge st that the apparent minor importance of Sip4 may be the result of autoregu latory crosstalk among the isofunctional activators Cat8 and Sip4. The dere pression deficiency of a CSRE-dependent reporter gene in a strain lacking t he Cat1 (Snf1) protein kinase can be suppressed by Sip4 fused to a strong h eterologous activation domain. This finding agrees with the idea that phosp horylation by Cat1 may convert Sip4 into a functional activator.