AfsR Recruits RNA Polymerase to the afsS Promoter: A Model for Transcriptional Activation by SARPs

AfsR, a protein belonging to the Streptomyces antibiotic regulatory protein (SARP) family, is a global regulator of secondary metabolism in Streptomyces coelicolor A3(2). AfsR consists of three major functional domains: an N-terminal SARP domain, a central ATPase domain, and a C-terminal tetratrico peptide repeat (TPR) domain. Two truncated AfsR proteins, AfsRΔTPR containing the SARP and ATPase domains and AfsRΔC containing only the SARP domain, exhibited the same DNA-binding specificity as that of full-length AfsR. Two monomers bound cooperatively to a direct repeat located eight nucleotides 5′ to the −10 element of the afsS promoter. Both truncated AfsR proteins, as well as full-length AfsR, were able to form ternary complexes with the afsS promoter and RNA polymerase (RNAP), although RNAP alone could not bind to the DNA. The DNA-(AfsRΔC)2-RNAP complex was capable of initiating afsS transcription in vitro, indicating that the ATPase and TPR domains are dispensable for the basic function of AfsR as a transcriptional activator. However, the ATPase domain was required to fully compensate for the defect in actinorhodin production in an afsR-disrupted mutant, suggesting that the ATPase domain exerts a regulatory function on the basic SARP domain. Deletion or addition of even a single nucleotide between the AfsR-binding site and the −10 element of the afsS promoter abolished afsS transcription both in vitro and in vivo, indicating that the recruitment of RNAP by AfsR to the correct location relative to the −10 element is critical for transcriptional activation. Since SARP-binding sites with similar direct repeats are located at the same position relative to the −10 element of their target promoters as is the afsS binding site, the SARP family members presumably activate transcription of their targets by recruiting RNAP to the promoter, where a ternary DNA–SARP–RNAP complex competent for transcriptional initiation is formed.