The phage phi 29 regulatory protein p4 activates the late promoter A3 by stabilizing the binding of Bacillus subtilis RNA polymerase (RNAP) as a closed complex. Interaction between the two proteins occurs through amino acid Arg120 in protein p4 and the C-terminal domain of the RNAP alpha subunit (alpha-CTD). In addition to its role as activator of the late transcription, protein p4 represses early transcription from the A2b and A2c promoters, that are divergently transcribed. Binding of p4 to its recognition site at the A3 promoter displaces the RNAP from promoter A2b, both by steric hindrance and by the curvature induced upon p4 binding. At the A2c promoter, the RNAP cooperates with p4 binding in such a way that promoter clearance is prevented. Interestingly, amino acid Arg120 in p4 and the alpha-CTD in B. subtilis RNAP are involved in the interactions that lead to transcription repression at promoter A2c. To investigate how this interaction leads to activation at PA3 and to repression at PA2c, mutant promoters were constructed. In the absence of a -35 consensus box for sigma A-RNAP activation was observed, while in its presence repression occurred. The results support the idea that overstabilization of RNAP at the promoter over a threshold level leads to repression.