Bacterial promoters differ in the number of RNA polymerase molecules that bind to form a filterable polymerase-promoter complex. We show that two holoenzyme molecules interact with the tyrT promoter, probably as a dimer. This interaction is inhibited by ppGpp. By contrast a single holoenzyme monomer suffices for complex formation at the lacUV5 promoter. We propose that In vivo promoter selection by monomeric and dimeric forms of the enzyme could coordinate the synthesis of stable RNA with that of mRNA and could also account in part for the switch in transcriptiona selectivity during the stringent response.