BINDING OF AN ESSENTIAL PLASMID PROTEIN TO THE DIRECTLY REPEATED DNA AT THE REPLICATIVE ORIGIN OF BROAD-HOST-RANGE PLASMID R1162

The R1162 replication protein RepIB binds to a 20 bp DNA sequence, rei terated 3 and 1/2 times within the replicative origin of the plasmid, with a stoichiometry of 2 protein monomers: 1 direct repeat. Insertion and deletion mutations in the repIB gene were isolated, and the alter ed proteins encoded by several of these purified and characterized. Mu tations in the amino-terminal coding region of the gene resulted in pr oteins that did not bind detectably to the direct repeat DNA. However, one of these proteins remained active for replication. Several of the non-binding proteins excluded R1162 from the cell. One of these inter fered with binding by normal protein when co-purified from the cell, b ut not when the two proteins were separately purified and mixed. Taken together, our results indicate that RepIB forms a stable dimer by an interaction at a region within the protein distinct from the DNA-bindi ng domain.