General activator of RNA polymerase III transcription. Factor exclusively required for RNA polymerase III transcription of genes with promoter elements upstream of the initiation sites. Contributes to the regulation of gene expression; functions as activator in the absence of oxidative stress. Down-regulates expression of target genes in response to oxidative stress. Overexpression protects cells against apoptosis in response to oxidative stress. Component of TFIIIB complexes. Interacts with TBP and forms a ternary complex with TBp and target DNA sequences. In response to oxidative stress, a Cys-residue is reversibly oxidized to cysteine sulfenic acid. This impairs formation of a ternary complex with TBP and DNA and down-regulates expression of target genes in response to oxidative stress. Belongs to the TFIIB family.