Pectobacterium carotovorum subsp. carotovorum strain Er simultaneously produces the phage tail-like bacteriocin carotovoricin (Ctv) and pectin lyase (Pnl) in response to DNA-damaging agents. The regulatory protein RdgB of the Mor/C family of proteins activates transcription of pnl through binding to the promoter. However, the optimal temperature for the synthesis of Ctv (23°C) differs from that for synthesis of Pnl (30°C), raising the question of whether RdgB directly activates ctv transcription. Here we report that RdgB directly regulates Ctv synthesis. Gel mobility shift assays demonstrated RdgB binding to the P 0 , P 1 , and P 2 promoters of the ctv operons, and DNase I footprinting determined RdgB-binding sequences (RdgB boxes) on these and on the pnl promoters. The RdgB box of the pnl promoter included a perfect 7-bp inverted repeat with high binding affinity to the regulator (K d [dissociation constant] ؍ 150 nM). In contrast, RdgB boxes of the ctv promoters contained an imperfect inverted repeat with two or three mismatches that consequently reduced binding affinity (K d ؍ 250 to 350 nM). Transcription of the rdgB and ctv genes was about doubled at 23°C compared with that at 30°C. In contrast, the amount of pnl transcription tripled at 30°C. Thus, the inverse synthesis of Ctv and Pnl as a function of temperature is apparently controlled at the transcriptional level, and reduced rdgB expression at 30°C obviously affected transcription from the ctv promoters with low-affinity RdgB boxes. Pathogenicity toward potato tubers was reduced in an rdgB knockout mutant, suggesting that the RdgAB system contributes to the pathogenicity of this bacterium, probably by activating pnl expression.Pectobacterium carotovorum subsp. carotovorum (formally Erwinia carotovora subsp. carotovora) is a causal agent of softrot disease in plants. This pathogen produces pectolytic enzymes, including pectate lyase, polygalacturonase, and pectin lyase (Pnl), which are crucial pathogenic determinants that can degrade pectin, a plant cell wall component (5). The regulatory mechanism underlying the synthesis of Pnl is exposure to DNA-damaging agents such as mitomycin C (MMC), nalidixic acid, and UV light (12,14,22,23,33). This differs from that underlying the synthesis of pectate lyase and polygalacturonase, the synthesis of which is stimulated by the substrate pectin (or pectic acid) through a quorum-sensing mechanism (5, 35). In P. carotovorum subsp. carotovorum strain Er and some other strains, DNA damage-inducible Pnl synthesis is accompanied by production of the bacteriocin carotovoricin (Ctv) (12,15,34,38). The Ctv of strain Er has a phage tail-like morphology consisting of a sheath, core, base plate, and several tail fibers (11,24). The Ctv genes (ctv) of strain Er cluster separately from the pnl locus on the chromosome, and they are transcribed in four units (36) (Fig. 1). The P 0 promoter precedes the lyt cassette, which contains the putative holin and murein hydrolase genes, responsible for cell lysis to liberate bacteriocin...