Efficient transcript production requires efficient release of RNA polymerase at the terminator; slow release at the terminator of a short transcription unit quickly becomes rate limiting for transcription at steady state. Although pol III autonomously recognizes sequence terminators, proteins that help to rapidly detach pol III from the terminator can affect the productivity of transcription if they eliminate termination as the rate-limiting step.
La, NF1 family proteins, PC4 and topoisomerase I have been proposed as accessory pol III transcription factors that facilitate multi-cycle transcription by hspol III, and are hence described as positive regulators of termination.
The biochemically purified NF1 family proteins exert strong effects on transcriptional termination and increase the number of rounds of transcription in vitro. Detailed analysis of this effect has been confined to the VAI gene, which harbors consensus NF1 binding sites that overlap its two terminators and are essential for the transcription termination activity of NF1. However, terminator-overlapping NF1 sites are not a general feature of hspol III-transcribed genes. Involvement of NF1 proteins in transcription of genes lacking NF1-binding sites, conceivably through interaction with TFIIIC2, has been referred to (Wang et al., 2000) but not presented in concrete detail. [Nor has the implicit conflict with the proposed essential character of NF1-binding sites at the VAI gene terminator been resolved.] Biochemically purified NF1 has been reported to exert no effect on transcription in a highly purified system consisting only of recombinant TFIIIB, immunopurified TFIIIC and pol III (Wang et al., 2000).