RNA polymerase III is one of three types of nuclear RNA polymerases present in eucaryotic cells. About 10% of the total transcription in dividing cells can be attributed to its activity. It synthesizes an eclectic collection of catalytic
RNA polymerase III is one of three types of nuclear RNA polymerases present in eucaryotic cells. About 10% of the total transcription in dividing cells can be attributed to its activity. It synthesizes an eclectic collection of catalytic or structural RNA molecules, some of which are involved in protein synthesis, pre-mRNA splicing, tRNA processing, and the control of RNA polymerase II elongation, whereas some others have still unknown functions. Like other RNA polymerases, RNA polymerase III cannot recognize its target promoters directly. Instead it is recruited to specific promoter sequences through the help of transcription factors. There are three basic types of RNA polymerase III promoters, called types 1, 2, and 3(Geiduschek and Kassavetis, 1992). Although in vivo, RNA polymerase III may be recruited to these promoters as part of a large complex (holo RNA polymerase III) containing the polymerase and its initiation factors (Wang et al., 1997), in vitro the reaction can be divided into several steps. First, the promoter elements are recognized by DNA binding factors, which then recruit a factor known as TFIIIB. TFIIIB itself then directly contacts RNA polymerase III. In human cells but not in S. cerevisiae, there are at least two versions of TFIIIB. One contains TBP, Bdp1, and Brf1 (Brf1-TFIIIB), and the other TBP, Bdp1, and Brf2 (Brf2-TFIIIB) (Schramm et al., 2000; Teichmann et al., 2000).