TRANSCRIPTION TERMINATION BY VACCINIA RNA-POLYMERASE ENTAILS RECOGNITION OF SPECIFIC PHOSPHATES IN THE NASCENT RNA

Vaccinia virus RNA polymerase terminates transcription downstream of a UUUUUNU signal in the nascent RNA. Transduction of the RNA signal to the elongating polymerase requires a termination factor (vaccinia term ination factor/capping enzyme) and is coupled to the hydrolysis of ATP . It was shown previously that incorporation of 5-bromouracil or 5-iod ouracil within the UUUUUNU element abolishes termination by preventing factor-dependent release of the nascent chain from the polymerase elo ngation complex. Here, we report that termination is prevented by phos phorothioate substitution at UMP residues in the nascent RNA. In contr ast, phosphorothioate substitution at AMP, CMP, and GMP nucleotides do es not inhibit termination. Thus, the action of a eukaryotic terminati on factor entails recognition of the nucleotide bases and the phosphat e groups of the target sequence in nascent RNA.