Comparison of Escherichia coli and T3 RNA Polymerases

The sensitivities of the Escherichia coli and bacteriophage T3 RNA polymerases to various compounds have been investigated in vitro. Neither enzyme is sensitive to the drug α‐amanitin. The E. coli polymerase is inhibited by an exotoxin from Bacillus thuringiensis ; the phage enzyme is not. Although both enzymes are inhibited by actinomycin D and distamycin A, there is a differential effect: the T3 enzyme is more sensitive to distamycin A than the E. coli enzyme, and less sensitive to actinomycin D. While distamycin A preferentially inhibits chain initiation by both enzymes, actinomycin D affects mainly chain elongation. The rifamycin derivative AF/013 was found to selectively inhibit chain initiation by the T3 RNA polymerase and this drug was used to characterize a transient initiation complex observed when the T3 RNA polymerase is incubated with GTP alone as a substrate.