在bp +5位点可以发生从中止转录到有效转录的转变。pol iii转录基因的主要转录本很短，90 ~ 120元的tRNA基因和5 sRNA(占绝大多数的产品),甚至最长的记录(如信号识别颗粒)的RNA只有~ 500元。特殊功能的转录起始的存在因素束缚在转录单位-内TFIIIA类1基因(NH部分)and TFIIIC in class 1 and class 2 genes. Transcript elongation, which has been analyzed at single-step resolution on a single sc tRNA gene, is rapid with highly purified components (TFIIIC, TFIIIB and pol III), and also in crude extracts (Matsuzaki et al., 1994; Shaaban et al., 1996). In particular, TFIIIC does not present a barrier to transcript elongation, generating a delay at only a single site of only 0.15-0.2 s duration at 20C. In other words, the requirement to displace TFIIIC during transcript elongation is not rate-limiting for tRNA gene activity. On the other hand, transcript elongation slows down when short U-tracts, e.g. UUU, are laid down, even at high concentrations of NTPs. These pauses exceed the TFIIIC-imposed delay on transcription of the sc SUP4 gene by a factor of ~3 or greater.Sc TFIIIC is entirely displaced from a tRNA gene during multiple cycles of transcription in vitro (Bardeleben et al., 1994) and its occupancy of pol III genes in vivo during active cell growth is also low (Roberts et al., 2003).
