In vitro analysis of elongation and termination by mutant RNA polymerases with altered termination behavior

We have studied the in vitro elongation and termination properties of several yeast RNA polymerase III (pol III) mutant enzymes that have altered in vivo termination behavior (S. A. Shaaban, B. M. Krupp, and B. D. Hall, Mol. Cell. Biol. 15:1467-1478, 1995). The pattern of completed-transcript release was also characterized for three of the mutant enzymes. The mutations studied occupy amino acid regions 300 to 325, 455 to 521, and 1061 to 1082 of the RET1 protein (P. James, S. Whelen, and B. D. Hall, J. Biol. Chem. 266:5616-5624, 1991), the second largest subunit of yeast RNA pol III. In general, mutant enzymes which have increased termination require a longer time to traverse a template gene than does wild-type pol III; the converse holds true for most decreased-termination mutants. One increased-termination mutant (K310T I324K) was faster and two reduced termination mutants (K512N and T455I E478K) were slower than the wild-type enzyme. In most cases, these changes in overall elongation kinetics can be accounted for by a correspondingly longer or shorter dwell time at pause sites within the SUP4 tRNA(Tyr) gene. Of the three mutants analyzed for RNA release, one (T455I) was similar to the wild type while the two others (T455I E478K and E478K) bound the completed SUP4 pre-tRNA more avidly. The results of this study support the view that termination is a multistep pathway in which several different regions of the RET1 protein are actively involved. Region 300 to 325 likely affects a step involved in RNA release, while the Rif homology region, amino acids 455 to 521, interacts with the nascent RNA 3' end. The dual effects of several mutations on both elongation kinetics and RNA release suggest that the protein motifs affected by them have multiple roles in the steps leading to transcription termination.