The primary event in gene transcription is the recognition of promoter sequences by transcription factors. One particularly interesting case is the binding of yeast transcription factor TFIIIC (z or tau) to tRNA genes. The elucidation of the molecular structure of X and the identification of the polypeptides that contact DNA are prerequisites to our understanding of the mechanism of tRNA gene activation. We have made some observations that hint at an important role for the 91 kDa subunit of TFIIIC (X91 ) in class III gene expression: It is the only TFIIIC subunit that can bind a nucleic acid on its own and there is a possible link between the transcription and RNA maturation processes (I have observed a strong RNA maturation defect in a z138 mutant that was suppressed in a n138+T91 double mutant). My research project aims at strengthening these observations and at establishing the role of T91 in TFIIIC. I shall first confirm that T91 iS part of TauB, the TFIIIC domain that binds to the B block of tRNA genes and has a predominant role in stabilizing T-DNA complex, then focus on the interactions engaged by T91 at the DNA, RNA or protein level.