Publisher Summary This chapter focuses on recent advances in the understanding of a critical limiting step in polymerase III (Pol III) transcription—the assembly on DNA of the initiation factor transcription factor IIIB (TFIIIB) by transcription factor IIIC (TFIIIC). The structural changes induced by Bdp1 are of particular interest because they lead to kinetic trapping of the DNA in TFIIIB and confer its characteristic high stability and resistance to dissociation by high salt concentrations and poly anions. The stability of TFIIIB-DNA permits high levels of transcription by eliminating rate-limiting steps in pre-initation complex assembly and by enabling rapid recycling of Pol III after termination. Complex stability is presumably also important for Pol III genes to function as boundaries that block the spread of silent chromatin. The significance of these interactions in Tfc4 may not be limited to its function in TFIIIB assembly; the association between Tfc4 and polymerase subunits raises the possibility of an additional role in post assembly functions for TFIIIC such as polymerase recruitment or recycling. As the list of effectors of Pol III transcription continues to expand in both yeast and mammalian cells, a common theme among known effectors can be recognized—the ability to target TFIIIB and affect its recruitment by TFIIIC or interactions with Pol III. In particular, the mechanisms by which posttranslational signaling events affect TATA-binding protein, Brf1, Bpd1, and TFIIIC function in TFIIIB assembly have yet to be defined.