Regulatory implications of protein assemblies at the gamma origin of plasmid R6K - a review

Recognition of the replication origin (ori) by initiator protein is a recurring theme for the regulated initiation of DNA replication in diverse biological systems. The objective of the work reviewed here is to understand the initiation process focusing specifically on the gamma-ori of the antibiotic-resistance plasmid R6K. The control of gamma-ori copy number is determined by both plasmid-encoded and host-encoded factors. The two central regulatory elements of the plasmid are a multifunctional initiator protein pi, and sequence-related DNA target sites, the inverted half-repeats (IRs) and the direct repeats (DRs). The replication activator and inhibitor activities of pi seem to be at least partially distributed between two naturally occurring pi polypeptides (designated by their molecular weights pi35.0 and pi30.5). Regulatory variants of pi with altered states of oligomerization in nucleoprotein complexes with DRs and IRs have been isolated. The properties of these mutants laid the foundation for our model of pi protein activity which proposes that different protein surfaces are required for the formation of functionally distinct complexes of pi with DRs and IRs. These mutants also suggest that pi polypeptides have a modular structure; the C-terminus contains the DNA-binding domain while the N-terminus controls protein oligomerization. Additionally, pi35.0 binds to a novel DNA sequence in the A+T-rich segment of gamma-ori. This binding site is at or near the site from which synthesis of the leading strand begins.