Genetic Composition of theBacillus subtilisSOS System

Abstract

The SOS response in bacteria includes a global transcriptional response to DNA damage. DNA damage is sensed by the highly conserved recombination protein RecA, which facilitates inactivation of the transcriptional repressor LexA. Inactivation of LexA causes induction (derepression) of genes of the LexA regulon, many of which are involved in DNA repair and survival after DNA damage. To identify potential RecA-LexAregulated genes in Bacillus subtilis, we searched the genome for putative LexA binding sites within 300 bp upstream of the start codons of all annotated open reading frames. We found 62 genes that could be regulated by putative LexA binding sites. Using mobility shift assays, we found that LexA binds specifically to DNA in the regulatory regions of 54 of these genes, which are organized in 34 putative operons. Using DNA microarray analyses, we found that 33 of the genes with LexA binding sites exhibit RecA-dependent induction by both mitomycin C and UV radiation. Among these 33 SOS genes, there are 22 distinct LexA binding sites preceding 18 putative operons. Alignment of the distinct LexA binding sites reveals an expanded consensus sequence for the B. subtilis operator: 5-CGAACATATGTTCG-3. Although the number of genes controlled by RecA and LexA in B. subtilis is similar to that of Escherichia coli, only eight B. subtilis RecA-dependent SOS genes have homologous counterparts in E. coli.Exposure of prokaryotes to DNA-damaging agents results in the induction of a diverse set of physiological responses collectively called the SOS response (8, 55). As first characterized in Escherichia coli, the SOS response includes an enhanced capacity for recombinational repair, enhanced capacity for excision repair, enhanced mutagenesis (due to error-prone repair), and inhibition of cell division (i.e., filamentation). Induction of the SOS response is due to the coordinate derepression of a number of SOS or din (for damage-inducible) genes. The SOS response to DNA damage in Bacillus subtilis is similar to that of E. coli (26,56,58), but unlike E. coli, the B. subtilis SOS system is also induced in competent cells in the absence of any DNA-damaging treatment (25, 57, 58). As in E. coli, SOS gene expression in B. subtilis is controlled by two proteins (which are themselves products of SOS genes): the LexA protein (also called DinR) (40,54), which represses the transcription of din genes by binding to the SOS operator (31), and the RecA protein (30), which is activated by single-stranded DNA (29,42) to stimulate the proteolytic autodigestion of LexA (24,31). Thus, an SOS gene is defined by two criteria-RecA-dependent induction by DNA damage and a binding site for LexA overlapping its promoter.By contrast with E. coli, where more than 30 SOS genes have been identified (7,8), only 5 B. subtilis SOS genes have been shown to meet both SOS gene criteria thus far: recA, lexA, uvrB (formerly dinA), dinB, and dinC (also called tagC) (4,9,15,25).

The SOS response of Listeria monocytogenes is involved in stress resistance and mutagenesis

“…This approach was previously used to identify the SOS regulons of E. coli (Courcelle et al, 2001), B. subtilis (Au et al, 2005;Goranov et al, 2006), Staphylococcus aureus (Cirz et al, 2007) and Pseudomonas aeruginosa (Cirz et al, 2006). The complete SOS regulon has furthermore been determined for Caulobacter crescentus (da Rocha et al, 2008) and Pseudomonas fluorescens (Jin et al, 2007).…”

“…It is regulated by LexA and RecA. LexA is an autoregulatory repressor that binds to the CGAACATATGTTCG consensus sequence in the promoter region of the SOSresponse genes as determined for Bacillus subtilis (Au et al, 2005), thereby repressing transcription. A consensus LexAbinding motif for L. monocytogenes has not been identified thus far.…”

The heat-shock response of Listeria monocytogenes comprises genes involved in heat shock, cell division, cell wall synthesis, and the SOS response

“…4). The LexA repressor is autoregulatory, and binds to the operator sequence in the promoter region of the SOS response genes (CGAACATATGTTCG in B. subtilis; Au et al, 2005). Several LexA-regulated genes showed increased expression in L. monocytogenes, including genes encoding the two alternative DNA polymerases umuDC and dinB.…”

Replication is required for the RecA localization response to DNA damage in Bacillus subtilis

“…In both E. coli and B. subtilis, DNA damage results in the induction of Ͼ40 unlinked genes that comprise the SOS response (43)(44)(45)(46). The recA gene is regulated by LexA, and recA transcript levels increase after exposure to exogenous damage (43).…”