In Saccharomyces cerevisiae, the cell surface protein, Muc1p, was shown to be critical for invasive growth and pseudohyphal differentiation. The transcription of MUC1 and of the co-regulated STA2 glucoamylase gene is controlled by the interplay of a multitude of regulators, including Ste12p, Tec1p, Flo8p, Msn1p and Mss11p. Genetic analysis suggests that Mss11p plays an essential role in this regulatory process and that it functions at the convergence of at least two signalling cascades, the filamentous growth MAPK cascade and the cAMP-PKA pathway. Despite this central role in the control of filamentous growth and starch metabolism, the exact molecular function of Mss11p is unknown. We subjected Mss11p to a detailed molecular analysis and report here on its role in transcriptional regulation, as well as on the identification of specific domains required to confer transcriptional activation in response to nutritional signals. We show that Mss11p contains two independent transactivation domains, one of which is a highly conserved sequence that is found in several proteins with unidentified function in mammalian and invertebrate organisms. We also identify conserved amino acids that are required for the activation function.