Reaction of the thiol groups of E. coli RNA polymerase with 7 chloro-4-nitrobenzo-2 oxa-1,3 diazole.

A single thiol group of $\text{E. Coli}$ RNA polymerase reacts rapidly with 7-chloro-4-nitrobenzo-2 oxa-1,3 diazole (NBD-C1) at pH 7.5 with slower reaction of a much larger number of such groups. Reaction of the unique thiol is associated with maximal loss of about 60% of the catalytic activity. The extent of inactivation is independent of the template used in the assay. Initiation of RNA synthesis shows no inhibition, thereby implicating the elongation step as the impaired function. Neither substrate, template, rifampcin, nor the elongation inhibitor rose bengal protect the enzyme against reaction with NBD-C1. These observations suggest that the partial inactivation of RNA polymerase on reaction with a thiol group is most likely due to a local conformational change outside of the active site which influences the active site in an indirect manner.