The alpha-amanitin domain or domain f of the largest subunit of RNA polymerases 
is one of the most conserved of these enzymes. We have found that the C-terminal 
part of domain f can be swapped between yeast RNA polymerase II and III. An 
extensive mutagenesis of domain f of C160, the largest subunit of RNA polymerase 
III, was carried out to better define its role and understand the mechanism 
through which C160 participates in transcription. One mutant enzyme, C160-270, 
showed much reduced transcription of a non-specific template at low DNA 
concentrations. Abortive synthesis of trinucleotides in a dinucleotide-primed 
reaction proceeded at roughly wild-type levels, indicating that the mutation did 
not affect the formation of the first phosphodiester bond, but rather the 
transition from abortive initiation to processive elongation. In specific 
transcription assays, on the SUP4 tRNA gene, pausing was extended but the rate 
of RNA elongation between pause sites was not affected. Finally, the rate of 
cleavage of nascent RNA transcripts by halted mutant RNA polymerase was 
increased approximately 10-fold. We propose that the domain f mutation affects 
the transition between two transcriptional modes, one being adopted during 
abortive transcription and at pause sites, the other during elongation between 
pause sites.