PROTEIN DOMAINS AND CONFORMATIONAL-CHANGES IN THE ACTIVATION OF REPA,A DNA-REPLICATION INITIATOR

RepA is the DNA replication initiator protein of the Pseudomonas plasm id pPS10. RepA has a dual function: as a dimer, it binds to an inverse ly-repeated sequence acting as a repressor of its own synthesis; as a monomer, RepA binds to four directly-repeated sequences to constitute a specialized nucleoprotein complex responsible for the initiation of DNA replication. We have previously shown that a Leucine Zipper-like m otif(LZ) at the N-terminus of RepA is responsible for protein dimeriza tion, In this paper we characterize the existence in RepA of two prote in globular domains C-terminal to the LZ, We propose that dissociation of RepA dimers into monomers results in a conformational change from a compact arrangement of both domains, competent for binding to the op erator, to an extended species that is suited for iteron binding. This model establishes the structural basis for the activation of DNA repl ication initiators in plasmids from Gramnegative bacteria.