Facilitated recycling pathway for RNA polymerase III.

We show that the high in vitro transcription efficiency of yeast RNA pol III is mainly due to rapid recycling. Kinetic analysis shows that RNA polymerase recycling on preassembled tDNA.TFIIIC.TFIIIB complexes is much faster than the initial transcription cycle. High efficiency of RNA pol III recycling is favored at high UTP concentrations and requires termination at the natural termination signal. Runoff transcription does not allow efficient recycling. The reinitiation process shows increased resistance to heparin as compared with the primary initiation cycle, as if RNA polymerase was not released after termination. Indeed, template competition assays show that RNA pol III is committed to reinitiate on the same gene. A model is proposed where the polymerase molecule is directly transferred from the termination site to the promoter.