We have examined the properties of two Drosophila RNA polymerase II mutants, C4 and S1, during elongation, pyrophosphorolysis, and DmS-II-stimulated transcript cleavage. The C4 and S1 mutants contain a single amino acid substitution in the largest and second largest subunits, respectively. Compared with wild type, C4 had a lower elongation rate and was less efficient at reading through intrinsic elongation blocks. S1 had a higher elongation rate than wild type and was more efficient at reading through the same blocks. During elongation, C4 and wild type responded similarly to DmS-II and NH, whereas the S1 mutant was less responsive to both. Differences between the two mutants also appeared during DmS-II-mediated transcript cleavage and pyrophosphorolysis. During extended pyrophosphorolysis, S1 polymerase was fastest and C4 polymerase was slowest at generating the final pattern of shortened transcripts. S1 and wild type were equal in the rate of extended DmS-II-mediated transcript cleavage, and C4 was slower. Our results suggest that the S1 mutation increases the time spent by the polymerase in elongation competent mode and that the C4 mutation may affect the movement of the polymerase.

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