RNA polymerase transcription factor IIF (TFIIF) is required for initiation at most, if not all, polymerase II promoters. We report here the cloning and sequencing of genes for a yeast protein that is the homolog of mammalian TFIIF. This yeast protein, previously designated factor g, contains two subunits, Tfg1 and Tfg2, both of which are required for transcription, essential for yeast cell viability, and whose sequences exhibit significant similarity to those of the mammalian factor. The yeast protein also contains a third subunit, Tfg3, which is less tightly associated and at most stimulatory to transcription, dispensable for cell viability, and has no known counterpart in mammalian TFIIF. Remarkably, the TFG3 gene encodes yeast TAF30, and furthermore, is identical to ANC1, a gene implicated in actin cytoskeletal function in vivo (Welch and Drubin 1994). Tfg3 is also a component of the recently described mediator complex (Kim et al. 1994), whose interaction with the carboxy-terminal repeat domain of RNA polymerase II enables transcriptional activation. Deletion of TFG3 results in diminished transcription in vivo.