The thumb subdomain of T7 RNA polymerase functions to stabilize the ternary complex during processive transcription

To examine the function of the thumb subdomain in bacteriophage T7 RNA polymerase we constructed a set of deletion mutants within this subdomain. These mutants exhibited reduced processivity during the processive, but not the abortive, stage of transcription. Reduced processivity was found to be due primarily to an increase in the processive ternary complex dissociation rate (destabilization of the processive ternary complex). The destabilization of the ternary complex does not appear to be due to a decrease in the affinity of the polymerase for the nascent RNA. These observations support the proposal that the thumb subdomain functions to stabilize the processive ternary complex during the processive (but not the abortive) stage of transcription, probably by wrapping around the template to prevent polymerase dissociation.