The transition from abortive to productive transcription may occur at bp +5. The primary transcripts of pol III-transcribed genes are short, ~90 to 120 nt for tRNA and 5 sRNA genes (which constitute the great majority of products) and even the longest transcripts (e.g. the RNA of the signal recognition particles) are only ~500 nt. The special feature of this transcription is the presence of initiation factors bound within the transcription unit ? TFIIIA within class 1 genes (NH section) 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).