Functional dissection of RNA polymerase III termination using a peptide nucleic acid as a transcriptional roadblock

We have shown previously that a T(10) peptide nucleic acid (PNA) bound to the transcriptional terminator of a Saccharomyces cerevisiae tDNA(Ile)(TAT) gene arrests elongating yeast RNA polymerase (pol) III at a position that precedes by 20 bp the upstream end of the PNA roadblock (Dieci, G., Corradini, R., Sforza, S., Marchelli, R., and Ottonello, S. (2001) J. Biol. Chem. 276, 5720-5725). Here, a PNA-binding cassette was placed at various distances downstream of a functional tDNA(Ile) transcriptional terminator (T(6)) that is not bound by the T(10) PNA, and the effect of the PNA roadblock on RNA 3'-end formation, transcript release, and transcription reinitiation was examined. With a PNA roadblock placed as close as 5 bp downstream of the T(6) terminator, pol III could still reach the termination site and complete pre-tRNA synthesis, implying that the catalytic site-to-front edge (C-F) distance of the polymerase can shorten by >10 bp upon recognition of the terminator element. In addition, transcripts synthesized by a PNA-roadblocked terminating pol III were found to be released from transcription complexes. Interestingly, however, the same roadblock dramatically reduced the rate of transcription reinitiation. Also, when placed 5 bp downstream of a mutationally inactivated terminator element (T(3)GT(2)), the PNA roadblock restored transcription termination, thus indicating that the inactivated terminator is compromised in its ability to cause pol III pausing, but can still induce C-F distance shortening and transcript release. The latter two activities were found to be further impaired in variants of the inactivated terminator bearing fewer than three consecutive T residues (T(2)G(2)T(2) and TG(2)TGT). The data indicate that RNA polymerase pausing, C-F distance shortening, and transcript release are functionally distinguishable features of the termination process and point to the RNA release propensity of pol III as a major determinant of its remarkably high termination efficiency.