FIG. 10.

Proposed transcriptional mechanism for regulation of genes encoding iron-using proteins as a function of iron availability in fission yeast. In the absence of iron, Fep1 is inactive and fails to repress php4⁺ gene expression. The Php4 that is synthesized forms a complex with Php2, Php3, and Php5, which represses expression of genes encoding iron-using proteins. In contrast, in the presence of iron, Fep1 interacts with GATA elements in the php4⁺ promoter to inactivate transcription. This inactivation of php4⁺ enables iron-using proteins to be expressed via the Php2/Php3/Php5 heterotrimeric CCAAT-binding factor.