Saturation mutagenesis of the E-coli RecA loop L2 homologous DNA pairing region reveals residues essential for recombination and recombinational repair

The disordered mobile loop L2 of the Escherichia coli RecA protein is known to play a central role in DNA binding and pairing. To investigate the loca l chemical environment in relation to function we performed saturation muta genesis of the loop L2 region (amino acid positions 193-212) using a site-d irected mutagenesis procedure, and determined the recombinational proficien cy of the 380 mutants using genetic assays for homologous recombination and recombinational repair. Residues Asn193, Gln194, Arg196, Glu207, Thr209, G ly211, and Gly212 were identified as stringently required for recombination al events in bacterial cells. In addition, our findings suggest the involve ment of loop L2 in the ATPase activity of RecA, and a role for residues Gln 194, Arg196, Lys198 and Thr209 in the DNA-dependent hydrolysis of ATP. Fina lly, since 20 residue peptides that comprise this region can pair homologou s DNAs by forming filamentous beta-structures, we propose how the informati on from the mutant analysis might facilitate the use of a simplified amino acid alphabet to design beta-structure forming L2 peptides with improved Re cA-like activities.