Functional divergence and potential mechanisms of the duplicate recA genes in Myxococcus xanthus

Abstract

RecA is a ubiquitous multifunctional protein for bacterial homologous recombination and SOS response activation. Myxococcus xanthus DK1622 possesses two recA genes, and their functions and mechanisms are almost unclear. Here, we showed that the transcription of recA1 (MXAN_1441) was less than one-tenth of recA2 (MXAN_1388). Expressions of the two recA genes were both induced by ultraviolet (UV) irradiation, but in different periods. Deletion of recA1 did not affect the growth, but significantly decreased the UV-irradiation survival, the homologous recombination ability, and the induction of the LexA-dependent SOS genes. Comparably, the deletion of recA2 markedly prolonged the lag phase for cellular growth and antioxidation of hydrogen peroxide, but did not change the UV-irradiation resistance and the SOS-gene inducibility. The two RecA proteins are both DNA-dependent ATPase enzymes. We demonstrated that RecA1, but not RecA2, had in vitro DNA recombination capacity and LexA-autolysis promotion activity. Transcriptomic analysis indicated that the duplicate RecA2 has evolved to mainly regulate the gene expressions for cellular transportation and antioxidation. We discuss the potential mechanisms for the functional divergence. This is the first time to clearly determine the divergent functions of duplicated recA genes in bacterial cells. The present results highlight that the functional divergence of RecA duplicates facilitates the exertion of multiple RecA functions. Author summary Myxobacteria has a large-size genome, contains many DNA repeats that are rare in the prokaryotic genome. It encodes bacterial RecA that could promote recombination between homologous DNA sequences. How myxobacteria avoid the undesired recombination between DNA repeats in its genome is an interesting question. M. xanthus encodes two RecA proteins, RecA1 (MXAN_1441) and RecA2 (MXAN_1388). Both RecA1 and RecA2 shows more than 60% sequence consistency with E. coli RecA (EcRecA) and can partly restore the UV resistance of E. coli recA mutant. Here, our results proved their divergent functions of the two RecAs. RecA1 retains the ability to catalyze DNA recombination, but its basal expression level is very low. RecA2 basal expression level is high, but no recombination activity is detected in vitro. This may be a strategy for M. xanthus to adapt to more repetitive sequences in its genome and avoid incorrect recombination. Highlights M. xanthus has two recAs, which are expressed with significantly different levels. Both recAs are inducible by UV irradiation, but in different stages. The absence of recA1 reduces bacterial UV-irradiation resistance, while the absence of recA2 delays bacterial growth and antioxidant capacity. RecA1 retains the DNA recombination and SOS induction abilities, while RecA2 has evolved to regulate the expression of genes for cellular transport and antioxidation.