Mol Gen Genet (1994) 245:523-527 © Springer-Verlag 1994 Joan Riera • Antonio R. Fern/mdez de Henestrosa Xavier Garriga • Angels Tapias - Jordi Barb6 Interspecies regulation of the recA gene of gram-negative bacteria lacking an E. coli-like SOS operator Received: 16 March 1994 / Accepted: 30 May 1994 Abstract The recA genes of Agrobacterium tumefaciens, Rhizobium meliloti, Rhizobium phaseoli and Rhodobacter sphaeroides, species belonging to the alpha-group bacte- ria of the Proteobacteria class, have been fused in vitro to the lacZ gene of Escherichia coli. By using a mini-Tn5 transposon derivative, each of these recA-lacZ fusions was introduced into the chromosome of each of the four species, and into that of E. coll. The recA genes of three of the alpha bacteria are induced by DNA damage when inserted in A. tumefaciens, R. phaseoli or R. rneliloti chromosomes. The expression of the recA gene of R. sphaeroides is DNA damage-mediated only when present in its own chromosome; none of the genes is induced in E. coli. Likewise, the recA gene of E. coli is not induced in any of the four alpha species. These data indicate that A. tumefaciens, R. meliloti and R. phaseoli possess a LexA-like repressor, which is able to block the expression of their recA genes, as well as that of R. sphaeroides, but not the recA gene of E. coli. The LexA repressor of R. sphaeroides does not repress the recA gene of A. tumefaciens, R. meliloti, R. phaseoli or E. coli. Introduction The SOS system of Escherichia coli includes at least 20 different genes, designated damage-inducible (din) or SOS genes, which are transcriptionally induced by a va- riety of treatments that damage the chromosome or in- hibit DNA replication (Little and Mount 1982; Walker 1984). The SOS response is controlled by the RecA and Communicated by R. Devoret J. Riera • A. R. Fernfindez de Henestrosa • X. Garriga A. Tapias • J. Barb6 ([~) Department of Genetics and Microbiology, Faculty of Sciences, Autonomous University of Barcelona, Bellaterra, 08193-Barcelona, Spain LexA proteins. LexA is the common repressor of all SOS genes, including both recA and lexA (Walker 1984). Interruption of replication or damage to the DNA gen- erates an induction signal that activates the constitutive level of RecA with the help of the Ssb proteins (Lieber- man and Witkin 1981; Weinstock and McEntee 1981); the activated RecA promotes cleavage of LexA, thus allowing transcription of SOS genes (Craig and Roberts 1980; Little et al. 1980). The characteristics of this in- ducing signal are still unknown, although it has been proposed that single-stranded DNA regions, which originate after DNA damage is processed during DNA replication (Sassanfar and Roberts 1990), may act as inducers. The term coprotease has recently been adopt- ed to designate the activated RecA protein, since this protein appears to play an indirect role in LexA cleav- age (Little 1991). There is evidence to suggest that SOS- like responses exist in other prokaryotes and, possibly, eukaryotes (Miller and Kokjohn 1990). The isolation and characterization of more than 40 recA genes from both gram-negative (Roca and Cox 1990) and gram- positive bacteria (Duwat et al. 1992), as well as 6 lexA genes from various species (Garriga et al. 1992; Ray- mond-Denise and Guillen 1991; Riera and Barb6 1993), have already been reported. The LexA repressor binds to a specific sequence (the SOS box), which functions as an operator site and is present upstream of the coding regions of the SOS genes. For E. coli, the consensus sequence of the SOS box is CTG-Nlo-CAG (Wertman and Mount 1985). It has been suggested that the major recognition element for the LexA repressor of E. coli comprises the CTGT motifs of the SOS box (Schnarr et al. 1991; Ottleben et al. 1991). The Bacillus subtilis SOS box seems to be GAAC-N4-GTTC (Cheo et al. 1991). Many of the recA genes isolated from gram-negative bacteria have an SOS box like that of E. coll. Nevertheless, some, such as those of Agrobacterium tumefaciens (Wardham et al. 1992), Thiobacillus ferrooxidans (Ramesar et al. 1989),