DNA-binding domain of the RepE initiator protein of mini-F plasmid: involvement of the carboxyl-terminal region.

The RepE initiator protein (251 residues) is essential for mini-F replication in Escherichia coli and exhibits two major functions: initiation of DNA replication from ori2 and autogenous repression of repE transcription. Whereas the initiation is mediated by RepE monomers that bind to the ori2 iterons (direct repeats), the autogenous repression is mediated by dimers that bind to the repE operator, which contains an inverted repeat sequence related to the iterons. We now report that the binding of RepE to these DNA sites is primarily determined by the C-terminal region of this protein. The mutant RepE proteins lacking either the N-terminal 33 (or more) residues or the C-terminal 7 (or more) residues were first shown to be defective in binding to both the ori2 and the operator DNAs. However, direct screening and analysis of mutant RepEs which are specifically affected in binding to the ori2 iterons revealed that the mutations (mostly amino acid substitutions) occur exclusively in the C-terminal region (residues 168 to 242). These mutant proteins exhibited reduced binding to ori2 and no detectable binding to the operator. Thus, whereas truncation of either end of RepE can destroy the DNA-binding activities, the C-terminal region appears to represent a primary DNA-binding domain of RepE for both ori2 and the operator. Analogous DNA-binding domains seem to be conserved among the initiator proteins of certain related plasmids.