Sok2 regulates yeast pseudohyphal differentiation via a transcription factor cascade that regulates cell-cell adhesion

In response to nitrogen limitation, Saccharomyces cerevisiae undergoes a di morphic transition to filamentous pseudohyphal growth. In previous studies, the transcription factor Sok2 was found to negatively regulate pseudohypha l differentiation. By genome array and Northern analysis, we found that gen es encoding the transcription factors Phd1, Ash1, and Swi5 were all induced in sok2/sok2 hyperfilamentous mutants. In accord with previous studies of others, Swi5 was required for ASH1 expression. Phd1 and Ash1 regulated expr ession of the cell surface protein Flo11, which is required for filamentous growth, and were largely required for filamentation of sok2/sok2 mutant st rains. These findings reveal that a complex transcription factor cascade re gulates filamentation. These findings also reveal a novel dual role for the transcription factor Swi5 in regulating filamentous growth. Finally, these studies illustrate how mother-daughter cell adhesion can be accomplished b y two distinct mechanisms: one involving Flo11 and the other involving regu lation of the endochitinase Cts1 and the endoglucanase Egt2 by Swi5.