Efg1 Directly Regulates ACE2 Expression To Mediate Cross Talk between the cAMP/PKA and RAM Pathways during Candida albicans Morphogenesis

The cyclic AMP/protein kinase A (cAMP/PKA) and regulation of Ace2 and morphogenesis (RAM) pathways are important regulators of the yeast-to-hypha transition in Candida albicans that interact genetically during this process. To further understand this interaction, we have characterized the expression of ACE2 during morphogenesis. In normoxic, planktonic conditions, ACE2 expression is very low in stationary-phase cells at both the mRNA and protein levels. Upon shifting to Spider medium, ACE2/Ace2p levels increase. Although Ace2 is not absolutely required for hypha formation, ace2 Delta/Delta mutants show delayed hypha formation in Spider medium (but not others) and morphological changes to the hyphal tip and lateral yeast. We also show that Efg1 directly binds the promoter of Ace2 in stationary phase, and ACE2 levels are increased in strains lacking Efg1 and the protein kinase A proteins Tpk1 and Tpk2, indicating that the PKA pathway directly regulates ACE2 expression. ACE2 expression is positively regulated by Tec1 and Brg1, which bind the promoters of ACE2 in hyphal cells but not in the yeast phase. Under embedded conditions, Efg1 is dispensable for filamentation and Ace2 is required. We have found that ACE2 expression is much higher in embedded cells than in planktonic cells, providing a potential rationale for this observation. Taken together, our observations indicate that the PKA pathway directly regulates the RAM pathway under specific conditions and are consistent with a model where the two pathways carry out similar functions that depend on the specific environmental context.