Dual targets of a transcriptional activator that tracks on DNA.

The sliding clamp of the bacteriophage T4 DNA polymerase, gp45, is also the proximal effector for activation of transcription of T4 late genes. We have identified the phage T4-encoded sigma factor gp55 and the co-activator gp33 as targets of gp45 in promoter complexes, and have shown that a conserved carboxy-terminal amino acid sequence of gp55 and gp33 is required for interaction with gp45. The respective contribution of each target-gp45 interaction to activation of transcription has been assessed by measuring promoter opening rates. The opening rate supported by interaction with both targets is far greater than the arithmetical sum of the separate contributions of each target, implying a synergistic activation of transcription through at least two separate interactions of the trimeric gp45.