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Binding of RecA protein to single‐stranded nucleic acids: spectroscopic studies using fluorescent polynucleotides.

Binding of the recA gene product from Escherichia coli to single‐stranded polynucleotides has been investigated using poly(dA) that have been modified by chloroacetaldehyde to yield fluorescent 1,N6‐ethenoadenine (epsilon A) bases. A strong enhancement of the fluorescent quantum yield of poly(d epsilon A) is induced upon RecA protein binding. A 4‐fold increase is observed in the absence of ATP or ATP gamma S and a 7‐fold increase in the presence of either nucleoside triphosphate. RecA protein can bind to poly(d epsilon A) in the absence of both Mg2+ ions and ATP (or ATP gamma S) but Mg2+ ions are required to observe RecA protein binding in the presence of ATP (or ATP gamma S) at pH 7.5. ATP binding to the RecA‐poly(d epsilon A) complex induces a dissociation of RecA from the polynucleotide followed by re‐binding of [RecA‐ATP‐Mg2+] ternary complex. Whereas ATP‐induced dissociation of RecA‐poly(d epsilon A) complexes is a fast process, the subsequent binding reaction of [RecA‐ATP‐Mg2+] is slow. A model is proposed whereby [RecA‐ATP‐Mg2+] binding to poly(d epsilon A) involves slow nucleation and elongation processes along the polynucleotide backbone. The nucleation reaction is shown to involve at least a trimer or a tetramer. Polymerization of the [RecA‐ATP‐Mg2+] ternary complex stops when the polynucleotide is entirely covered with 6 +/‐ 1 nucleotides per RecA monomer. ATP hydrolysis then induces a release of RecA‐ADP complexes from the polynucleotide template.