Activation of transcription initiation by the cl protein of phage lambda is thought to be mediated by a direct interaction between cl and RNA polymerase at the P(RM) promoter. Two negatively charged amino acid residues in the DNA binding domain of cl play a key role in activation, suggesting that these residues contact RNA polymerase. The subunit of RNA polymerase involved was identified by selecting polymerase mutants that restored the activation function of a mutant form of cl protein. Although previous studies suggest that several activators interact with the alpha subunit of RNA polymerase, the results here suggest that cl interacts with the sigma subunit. An arginine to histidine change near the carboxyl terminus of sigma specifically suppresses an aspartic acid to asparagine change in the activation region of cl. This finding supports the direct-contact model and suggests that a cluster of positively charged residues near the carboxyl terminus of sigma is the target of the negatively charged activation region of cl.