Control of white-opaque phenotypic switching in Candida albicans by the Efg1p morphogenetic regulator

Phenotypic switching in Candida albicans spontaneously generates different cellular morphologies and is manifested in strain WO-1 by the reversible sw itching between the white and opaque phenotypes, We present evidence that p henotypic switching is regulated by the Efg1 protein, which is known as an essential element of hyphal development (dimorphism). Firstly, EFG1 is expr essed specifically in cells of the white but not the opaque phenotype, Duri ng mass conversion from the opaque to the white phenotype, the EFG1 transcr ipt level correlates with competence of switching of opaque cells to the wh ite form. Secondly, overexpression of EFG1 by a PCK1p-EFG1 fusion forces op aque-phase cells to switch to the white form with a high level of efficienc y, Thirdly, low-level expression of EFG1 in strain CAI-8 generates a cellul ar phenotype similar to that of opaque cells in that cells bud as short rod s, which cannot be induced to form hyphae in standard conditions; such cell s (unlike authentic opaque cells) lack typical surface "pimples." Important ly, the opaque-specific OP4 transcript is induced in the opaque-like cells generated by strain CAI8 as a response to low-level expression of EFG1, The results suggest that high EFG1 expression levels induce and maintain the w hite cell form while low EFG1 expression levels induce and maintain the opa que cell form. It is proposed that changes in EFG1 expression determine or contribute to phenotypic switching events in C, albicans.