Assembly of RecA subunits into long, helical oligomers is required for its roles in recombinational DNA repair and homologous genetic recombination. The crystal structure of RecA reveals an extensive network of amino acid residues that lie at the subunit boundaries. We have introduced a large set of substitutions at 5 clustered residues, which are shown in the crystal structure to make specific contacts with positions in the neighboring monomer. We find that 3 of the 5 residues are important for RecA function (Lys216, Phe217, and Arg222), whereas the other 2 (Asn213 and Tyr218) are not. The patterns of functionally allowed substitutions provide insight into the chemical and steric constraints required at these positions.