Candida albicans Double Mutants Lacking both EFG1 and WOR1 Can Still Switch to Opaque

The switch from white to opaque in Candida albicans was discovered 33 years ago, but it is still unclear how it is regulated. A regulatory paradigm has emerged in which two transacting factors, Efg1 and Wor1, play central roles, Efg1 as a repressor of WOR1 , which encodes an activator of the transition to the opaque phenotype. However, we show here that if both EFG1 and WOR1 are deleted simultaneously, bona fide opaque cells can still be induced en masse . These results are not compatible with the simple paradigm, suggesting that an alternative opaque pathway (AOP) exists, which can activate expression of opaque and, like WOR1 , is repressed by EFG1 .