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Journal of Bacteriology, August 2005, p. 5367-5375, Vol. 187, No. 15
0021-9193/05/$08.00+0     doi:10.1128/JB.187.15.5367-5375.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Expression of Canonical SOS Genes Is Not under LexA Repression in Bdellovibrio bacteriovorus

Centre de Recerca en Sanitat Animal (CReSA), 08193 Bellaterra, Spain,¹ Departament de Genètica i Microbiologia, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain,² Biomedical Applications Group, Centro Nacional de Microelectrónica, 08193 Bellaterra, Spain³

Received 23 March 2005/ Accepted 9 May 2005

The here-reported identification of the LexA-binding sequence of Bdellovibrio bacteriovorus, a bacterial predator belonging to the -Proteobacteria, has made possible a detailed study of its LexA regulatory network. Surprisingly, only the lexA gene and a multiple gene cassette including dinP and dnaE homologues are regulated by the LexA protein in this bacterium. In vivo expression analyses have confirmed that this gene cassette indeed forms a polycistronic unit that, like the lexA gene, is DNA damage inducible in B. bacteriovorus. Conversely, genes such as recA, uvrA, ruvCAB, and ssb, which constitute the canonical core of the Proteobacteria SOS system, are not repressed by the LexA protein in this organism, hinting at a persistent selective pressure to maintain both the lexA gene and its regulation on the reported multiple gene cassette. In turn, in vitro experiments show that the B. bacteriovorus LexA-binding sequence is not recognized by other -Proteobacteria LexA proteins but binds to the cyanobacterial LexA repressor. This places B. bacteriovorus LexA at the base of the -Proteobacteria LexA family, revealing a high degree of conservation in the LexA regulatory sequence prior to the diversification and specialization seen in deeper groups of the Proteobacteria phylum.

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