Productive transcription is accompanied by retractive RNase activity at U-tract pause sites and at the terminator. The principal cleavage products are dinucleotides, and they are produced in large stoichiometric excess over complete transcripts, despite the rapid overall rate of productive RNA chain elongation (Bobkova and Hall, 1997). This RNase activity is intrinsic to pol III, and is mediated by its C11 subunit (gene RPC10), a paralog of pol II Rpb9 with a C-terminal 26 amino acid domain that is highly similar to the extrinsic pol II transcription factor TFIIS (Chédin et al., 1998). Two acidic amino acids in this segment (D91/E92 in scC11; D88/E89 in hC11), which are universally conserved in bacterial, archaeal and eukaryotic transcript cleavage factors, play a critical role in the activation of transcript cleavage by donating catalytic residues to the active centers of RNA polymerases for hydrolytic RNA chain retraction (Chédin et al., 1998; Kettenberger et al., 2003; Opalka et al., 2003; Sosunova et al., 2003). However, the extrinsic transcript cleavage factors, as well as the C11 subunit of pol III, are devoid of separate RNase activity.