The LexA regulon encompasses an ensemble of genes involved in preserving cell viability under massive DNA damage and is present in most bacterial phyla. Up to date, however, the scope of this network had only been assessed in the Gamma Proteobacteria. Here, we report the structure of the LexA regulon in the Alpha Proteobacteria, using a combined approach that makes use of in vitro and in vivo techniques to assist and validate the comparative genomics in silico methodology. This leads to the first experimentally validated description of the LexA regulon in the Alpha Proteobacteria, and comparison of regulon core structures in both classes suggests that a least common multiple set of genes (recA, ssb, uvrA and ruvCAB) might be a defining property of the Proteobacteria LexA network.