Diarrheic food poisoning by bacteria of the Bacillus cereus group is mostly due to several toxins encoded in the genomes. One of them, cytotoxin K, was recently identified as responsible for severe necrotic syndromes. Cytotoxin K is similar to a class of proteins encoded by genes usually annotated as haemolysin II (hlyII) in the majority of genomes of the B. cereus group. The partially sequenced genome of Bacillus thuringiensis var israelensis ATCC35646 contains several potentially induced prophages, one of them integrated into the hlyII gene. We determined the complete sequence and established the genomic organization of this prophage-designated phIS3501. During induction of excision of this prophage with mitomycin C, intact hlyII gene is formed, thus providing to cells a genetic ability to synthesize the active toxin. Therefore, this prophage, upon its excision, can be implicated in the regulation of synthesis of the active toxin and thus in the virulence of bacterial host. A generality of selection for such systems in bacterial pathogens is indicated by the similarity of this genetic arrangement to that of Staphylococcus aureus？？ -haemolysin. 1. Introduction Many bacterial strains of the B. cereus group are pathogenic to different eukaryotic organisms, including animals, insects, and nematodes [1–6]. The caused illnesses are mainly attributed to the synthesis of toxins and protective cellular structures, usually encoded by plasmids or, in the cases of diarrheic food intoxications, in the chromosome. In addition to the importance of plasmids, carrying the toxins, it was also suggested that the temperate phages can be involved in the adaptation of these bacteria to animal hosts [1, 7]. It is indicative in this respect that several sequenced genomes of these bacteria possess large extrachromosomal elements encoding plasmid-related and phage-related functions [1, 5, 8–12]. The pathogen evolution can thus be regarded as a constant dynamic exchange of genes between plasmids and temperate phages integrated or not into the bacterial chromosome. A notable illustration of such prophage-plasmid coevolution is the similarity between the genome of a large phage 0305f8-36, isolated from B. thuringiensis and a contig of genome of the strain B. weihenstephanensis KBAB4 [13, 14]. In fact, this contig corresponds to the 417？kb extrachromosomal element pBWB401 and could therefore be regarded as either a plasmid or a nonintegrated prophage. Still no evidence exists for the association between phages and toxicity genes in the B. cereus group, although the presence in
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