Activation of rho dependent transcription termination by NusG - Dependenceon terminator location and acceleration of RNA release

There is a kinetic limitation to Rho function at the first intragenic termi nator in the lacZ gene (tiZ1) which can be overcome by NusG:Rho can termina te transcription with slowly moving, but not rapidly moving, RNA polymerase unless NusG is also present. Here we report further studies with two other Rho dependent terminators that are not kinetically limited (tiZ2 and lambd a tR1) which show that the requirement for NusG depends on the properties o f the terminator and its location in the transcription unit. NusG is also s hown to increase the rate of Rho-mediated dissociation of transcription com plexes arrested at a specific termination stop point in the tiZ1 region and the rates of dissociation with three different Rho factors and two differe nt terminators correlated with their sensitivity to RNA polymerase elongati on kinetics. These results suggest a model of NusG function which involves an alteration in the susceptibility of the transcription complex to Rho act ion which allows termination to occur within the short kinetic window when RNA polymerase is traversing the termination region.