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The basic replicon of plasmid pCU1 contains three different replication origins. Replication initiated from the oriB origin requires pCU1-encoded protein RepA. Previously, information analysis of 19 natural RepA binding sequences predicted a 20-bp sequence as a RepA binding site. Guanines contacting RepA in the major groove of DNA have also been determined. In this study, we used the missing-nucleoside method to determine all of the bases relevant to RepA binding. The importance of some thymine bases was also confirmed by a missing-thymine site interference assay. Participation of the 5-methyl groups of two thymines (at positions -6 and 7) in RepA binding was pointed out by a missing-thymine methyl site interference assay. Phosphate groups of the DNA backbone which strongly interfered with RepA binding upon ethylation were also identified. The pattern of contacting positions mapped by hydroxyl radical protection footprinting indicates that RepA binds to one face of B-form DNA. The length of the binding site was found to be 20 bp by dissociation rate measurement of complexes formed between RepA and a variety of binding sequences. The symmetry of the binding site and that of the contacting bases, particularly the reacting 5-methyl groups of two thymines, suggest that pCU1-encoded RepA may contact its site as a homodimer.

A 2kb DNA region of the broad-host-range plasmid pCU1 carries all of the information essential for the stable maintenance of the plasmid and to express the same host-range specificity. It was predicted that the protein required to initiate replication from at least one of the three origins of the plasmid is encoded by the longest open-reading frame (ORF239) of the three overlapping in-frame ORFs located within the 2 kb region. The product of ORF239 has been named RepA. The initiator protein was overexpressed, purified and used for in vitro binding studies. Gel mobility shift experiments were performed to localize RepA binding sites. The DNA sequence protected by the bound RepA molecule(s) was determined by DNase I footprinting and 19 of a 20 bp long sequence that is part of the protected sequence were located in two clusters flanking the repA gene. A plasmid created by linking a 310 bp fragment (nucleotides 238 to 547) of the 2 kb region to the antibiotic resistance genes carried by the omega fragment, can be maintained stably if the RepA protein is supplied in trans. We conclude that this 310 bp DNA fragment, which consists of a short G+C and a long A+T rich region and the cluster of five RepA binding sites, carries a functional origin of the plasmid-protein dependent replication. The position of this origin indicates that it is oriB, one of the three origins previously identified by electron microscopy. The second cluster of RepA binding sites is downstream of the repA gene and consists of 14 sites that are in inverted orientation compared with the binding sites located in the oriB region. They are part of the region that was shown formerly to be involved in controlling the copy number of the plasmid. In contrast to oriB, binding of RepA to neither the oriS nor oriV region was detected.

RepA, the replication initiator protein of plasmid P1, binds to specific 19 bp sequences on the plasmid DNA. Earlier footprinting studies with dimethylsulfate identified the guanines that contact RepA through the major groove of DNA. In this study, base elimination was used to identify the contribution of all four bases to the binding reaction. Depurination and depyrimidation of any base in the neighborhood of the contacting guanines was found to decrease RepA binding. These results are consistent with the notion that RepA contacts bases of two consecutive major grooves on the same face of DNA. We also observed that depurination but not methylation of three guanines (G3, G8 and G9) affected binding. We identified the DNA phosphate groups (3 in the top strand, one of which mapped between G8 and G9, and 4 in the bottom strand, one of which was adjacent to C3) that strongly interfered with RepA binding upon ethylation. These results indicate that certain bases (e.g. G3, G8 and G9) may not contact RepA directly but contribute to base and backbone contacts by maintaining proper structure of the binding site.

The replication initiator protein RepA of plasmid P1 can bind to 14 sites on the plasmid. These sites are variously used to autoregulate RepA synthesis and for initiation and control of DNA replication. Analysis of information (degree of conservation) at the sites revealed three sequence patches of high conservation. By saturation mutagenesis, the conservation at the outer two patches was found to contribute to RepA binding more critically. The guanine bases that are likely to contact RepA through the major groove were identified by methylation interference and methylation protection experiments. These bases mapped to the outer two patches and were separated by one turn of the helix. Therefore, they belong to major grooves on the same face of DNA. All backbone contacts of the protein, determined by hydroxyl radical footprinting, also mapped to the same face. We conclude from this that RepA binds to its site on one face of the DNA. Information analysis of binding sites for several prokaryotic repressors and activators, where the nature of DNA-protein contacts are known, revealed a correlation between the positions of high conservation and the positions of major grooves that faced the protein. The middle patch of high conservation in the RepA binding sites is an exception since in this region a minor groove is likely to face the protein. The simplest model for minor groove contacts suggests that in B-form DNA a T.A base-pair cannot easily be distinguished from an A.T pair by inspection of the minor groove. Yet in the RepA site, a T-->A mutation in the middle patch significantly affects binding. Therefore, the simplest models for both minor and major groove contacts are unlikely. It is possible that the patch determines the proper conformation of the site and thereby contributes to recognition indirectly.

RepA protein of plasmid P1 binds to arrays of 19 bp repeat sequences (iterons) and mediates initiation of replication and its control. Escherichia coli heat shock proteins DnaJ and DnaK can stimulate iteron binding activity of RepA in an ATP-dependent fashion. It has been proposed that RepA binds to DNA as monomers and that the stimulation in binding involves monomerization of RepA dimers which are inactive in the binding reaction. RepA-iteron and RepA-RepA interactions have been measured in this study to determine the equilibrium constants of the two reactions. The apparent KD value for RepA-iteron binding decreased from 10 nM to no more than 0.2 nM at increasing concentrations of the heat shock proteins. The stimulation of binding appears to be due to an increase in active RepA fraction and not to a change in the maximum binding capacity of the active species. This view was deduced from measurements of active RepA fraction, which increased in the presence of heat shock proteins, and from measurements of dissociation rate constants, which were independent of the heat shock protein concentrations. Accounting for the active fractions, the true KD value was estimated to be 0.10(+/- 0.09) nM in 20 mM Tris.HCl (pH 8), 100 mM NaCl, 40 mM KCl, 10 mM MgCl2, 1 mM dithiothreitol, 0.1 mM EDTA, ATP (50 microM), bovine serum albumin (50 micrograms/ml), calf thymus DNA (50 micrograms/ml) and glycerol (5%). The dissociation rate constant was 1.5 x 10(-2) s-1 and the calculated association rate constant was 1.5 x 10(8) M-1 s-1. Ultracentrifugation analyses of RepA at 15,000 r.p.m. in the above buffer but without ATP, bovine serum albumin, calf thymus DNA and glycerol, revealed that the protein was in monomer-dimer equilibrium with a KD of 2.6(+/- 0.2) microM at 5 degrees C. Therefore, at protein concentrations used in the binding reactions, RepA is monomeric (> 99.5%), in confirmation of the earlier result that RepA binds as a monomer. It follows that the species that is stimulated to bind by the heat shock proteins is also a monomeric form of RepA.