The carbon source-responsive element (CSRE) functions as an activating promoter motif of gluconeogenic genes in Saccharomyces cerevisiae. The positively acting regulatory genes CAT8 and SIP4 encode CSRE-binding proteins which contribute unequally to the regulated expression of a CSRE-dependent reporter gene (85% and 15%, respectively, under conditions of glucose derepression). 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 other protein, arguing against the physiological significance of heterodimer formation. Gel retardation assays provide evidence for a different binding affinity of Cat8 and Sip4 to at least some CSRE sequence variants. Both efficient biosynthesis of and transcriptional activation by Sip4 require a functional CAT8 gene, while Cat8 was not dependent on SIP4. Thus, our data suggest that the apparent minor importance of Sip4 may be the result of autoregulatory cross-talk among the isofunctional activators Cat8 and Sip4. The derepression deficiency of a CSRE-dependent reporter gene in a strain lacking the Cat1 (Snf1) protein kinase can be suppressed by Sip4 fused to a strong heterologous activation domain. This finding agrees with the idea that phosphorylation by Cat1 may convert Sip4 into a functional activator.