Control of pseudohyphae formation in Saccharomyces cerevisiae

Pseudohyphal growth in both haploid and diploid strains of Saccharomyces ce revisiae reflects concerted changes in different cellular processes: buddin g pattern, cell elongation and cell adhesion. These changes are triggered b y environmental signals and are controlled by several pathways which act in parallel. Nitrogen deprivation, and possibly other stresses, activate a MA P kinase cascade which has the transcription factor Ste12 as its final targ et. A cAMP-dependent pathway, in which the protein kinase Tpk2 plays a spec ific role, is also required for the morphogenetic switch. Both pathways con tribute to modulate the expression of the MUC1/FLO11 gene which encodes a c ell-surface flocculin required for pseudohyphal and invasive growth. The MA P kinase cascade could also control the activity of the cyclin/ Cdc28 compl exes which affect both the budding pattern of yeast and cell elongation. A further protein which stimulates filamentous growth in S, cerevisiae is Phd 1, although its mode of action is unknown, it may be regulated by a cAMP-de pendent protein kinase, as occurs with the homologous protein Efg1 from Can dida albicans, which is required for the formation of true hyphae. Morphoge nesis in different yeast genera share common elements, but there are also i mportant differences. Although a complete picture cannot yet be drawn, part ial models may be proposed for the interaction of the regulatory pathways, both in the case of S. cerevisiae and in that of C. albicans. (C) 2001 Fede ration of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.