Mechanistic
analysis of RNA polymerase III hyper-processivity.
Roberto Ferrari, Claudio Rivetti, Giorgio Dieci
Dept. Biochem. and Mol. Biol., University of Parma, Area delle Scienze, Parma,
43100, Italy (gdieci@unipr.it)
Eukaryotic RNA polymerase (Pol) III is recruited to its target promoters by a stable preinitiation complex containing transcription factors TFIIIC and TFIIIB. After the first transcription cycle, reinitiation by Pol III proceeds through iterated, fast recruitment of the enzyme by the same transcription unit (hyper-processivity). RNA polymerase III hyper-processivity might simply be the result of an increased probability of enzyme re-attachment to the same gene, due to the close proximity of initiation and termination sites in the small-sized class III genes. Using purified transcription components from Saccharomyces cerevisiae, we show here that RNA polymerase III retention by the same gene after termination occurs in vitro even in the presence of a juxtaposed competitor gene, while it is lost when transcription is carried out in a simplified, factor-independent system. Facilitated Pol III recycling is thus not a purely stochastic process, but it rather depends on specific macromolecular contacts within the RNA polymerase III transcription complex. The nature of such interactions is currently being investigated using both TFIIIC-dependent and TFIIIC-independent templates of different lengths.