The faulty SOS response of Pseudomonas putida KT2440 stems from an inefficient RecA-LexA interplay

Despite its environmental robustness Pseudomonas putida strain KT2440 is very sensitive to DNA damage and displays poor homologous recombination efficiencies. To gain an insight into this deficiency isogenic ?recA and ?lexA1 derivatives of prophage-free strain P. putida EM173 were generated and responses of the recA and lexA1 promoters to DNA damage tested with GFP reporter technology. Basal expression of recA and lexA1 of P. putida were high in the absence of DNA damage and only moderately induced by norfloxacin. A similar behaviour was observed when equivalent GFP fusions to the recA and lexA promoters of E. coli were placed in P. putida EM173. In contrast, all SOS promoters were subject to strong repression in E. coli, which was released only when cells were treated with the antibiotic. Replacement of P. putida's native LexA1 and RecA by E. coli homologues did not improve the responsiveness of the indigenous functions to DNA damage. Taken together, it seems that P. putida fails to mount a strong SOS response due to the inefficacy of the crucial RecA-LexA interplay largely tractable to the weakness of the corresponding promoters and the inability of the repressor to shut them down entirely in the absence of DNA damage.