The regulation of EFG1 in white–opaque switching in Candida albicans involves overlapping promoters

Summary EFG1, which encodes atrans‐acting factor, is expressed as a more abundant 3.2 kb transcript in the white phase and as a less abundant 2.2 kb transcript in the opaque phase of the white–opaque transition inCandida albicans. To understand how alternative phase‐specific mRNAs are transcribed from the same gene locus, the 2320 bp upstream region of the gene was functionally characterized by analysing the ­activity of deletion derivatives in a luciferase‐based reporter system. The white phase‐specific promoter contained three discrete sequences involved in white phase‐specific activation, between −2022 and −1809 bp (AR1), between −1809 and −1727 bp (AR2) and between −922 and −840 bp (AR3). A higher resolution deletion and mutation analysis of AR2 revealed two regions between −1809 and −1787 bp and between −1764 and −1728 bp that are responsible for AR2 activation. Targeting of promoter constructs to the ectopicADE2genomic site and the 3′ end of theEFG1genomic site revealed a positional requirement for white phase‐regulated activation specific for the AR2 region of the promoter. Gel mobility shift assays using AR2 revealed a white phase‐specific activation complex. No discrete activation sequences were identified in the overlapping promoter of the opaque phase‐specificEFG1 transcript. The strength of opaque phase activation was directly proportional to the length of the promoter. Northern analysis excluded the possibility of an opaque phase‐specific repressor. These results demonstrate overlapping promoters for white and opaque phase‐specific expression of the gene for the transcription factor Efg1, with distinctly different mechanisms of phase‐specific activation.