Genetic recombinationparticipates in multiple physiological pathways that are crucial for themaintenance and diversification of the genome. The recombinase RecA is acentral factor in this process, mediating the exchange between DNA moleculescontaining perfect homology (homologous, HO) or very low divergence(homeologous, HE). At present multiple mechanisms are known to mediate geneticrecombination, both dependent and independent of RecA. In order to study therecombination process in Gram-negative bacteria, we used a LacZ based system to determine both HO andHE recombination. Using this system, we determined the recombination rates in wild-type(WT) and RecA deficient (ΔrecA) strains of Pseudomonasaeruginosa and Escherichiacoli. Moreover, the recombination process creates a functional copy of the lacZ gene and thus, the recombinantclones can be detected by their β galactosidase activity.

 In both bacteria, HO recombination rates wereapproximately 60-200-fold higher than HE recombination rates. In the RecA-deficientE. coli strain, both HO and HErecombination rates were 60-fold lower than that obtained for the WT strain.The recA deletion mutant ofP. aeruginosa showed that HO andHE recombination rates decreased 15 and 3-fold respectively, compared with theWT strain. When the β galactosidase activity of recombined clones wasdetermined, we found that all E. coli clones showed enzymatic activityas it was expected. In P. aeruginosahowever, many clones showed no β galactosidase activity. Furthermore, the molecularanalysis of the recombined regions of these clones showed the presence of pointmutations and also deletions of as much as 800bp.These results indicate theexistence of a significantly mutagenic recombination mechanism, independent ofRecA in P. aeruginosa, at differenceof E. coli.