Many transcriptional activators in prokaryotes are known to bind near a promoter and contact RNA polymerase, but it is not clear whether a protein-protein contact between an activator and RNA polymerase is enough to activate gene transcription. Here we show that contact between a DNA-bound protein and a heterologous protein domain fused to RNA polymerase can elicit transcriptional activation; moreover, the strength of this engineered protein-protein interaction determines the amount of gene activation. Our results indicate that an arbitrary interaction between a DNA-bound protein and RNA polymerase can activate transcription. We also find that when the DNA-bound 'activator' makes contact with two different components of the polymerase, the effect of these two interactions on transcription is synergistic.