Running Title: Evolution of the LexA binding sequence

Abstract

In recent years, the recognition sequence of the SOS repressor LexA protein has been identified for several bacterial clades, such as the Gram-positive, Green-non Sulfur bacteria and Cyanobacteria phyla, or the Alpha, Delta and Gamma Proteobacteria classes. Nevertheless, the evolutionary relationship among these sequences and the proteins that recognize them has not been analyzed. Fibrobacter succinogenes is an anaerobic Gram-negative bacterium that branched from a common bacterial ancestor immediately before the Proteobacteria phylum. Taking advantage of its intermediate position in the phylogenetic tree, and in an effort to reconstruct the evolutionary history of LexA binding sequences, the F. succinogenes lexA gene has been isolated and its product purified to identify its DNA recognition motif through electrophoretic mobility assays and footprinting experiments. After comparing the available LexA DNA binding sequences with the here reported F. succinogenes one, directed mutagenesis of the F. succinogenes LexA binding sequence and phylogenetic analyses of LexA proteins have revealed the existence of two independent evolutionary lanes for the LexA recognition motif that emerged from the Gram