The genome sequence ofClostridium botulinumtype C neurotoxin-converting phage and the molecular mechanisms of unstable lysogeny

Authors:
  • KKKen Kurokawa
    Ken Kurokawa
  • KNKeisuke Nakayama
    Keisuke Nakayama
  • KOKenshiro Oshima
    Kenshiro Oshima
  • YFYukako Fujinaga
    Yukako Fujinaga
  • MOMakoto Ohnishi
    Makoto Ohnishi
  • EOEiichi Ohtsubo
    Eiichi Ohtsubo
  • MHMasahira Hattori
    Masahira Hattori
  • KOKeiji Oguma
    Keiji Oguma
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Abstract
Botulinum neurotoxins (BoNTXs) produced byClostridium botulinumare among the most poisonous substances known. Of the seven types of BoNTXs, genes for type C1 and D toxins (BoNTX/C1 and D) are carried by bacteriophages. The gene for exoenzyme C3 also resides on these phages. Here, we present the complete genome sequence of c-st, a representative of BoNTX/C1-converting phages. The genome is a linear double-stranded DNA of 185,682 bp with 404-bp terminal direct repeats, the largest known temperate phage genome. We identified 198 potential protein-coding regions, including the genes for production of BoNTX/C1 and exoenzyme C3. Very exceptionally, as a viable bacteriophage, a number of insertion sequences were found on the c-st genome. By analyzing the molecular structure of the c-st genome in lysogens, we also found that it exists as a circular plasmid prophage. These features account for the unstable lysogeny of BoNTX phages, which has historically been called “pseudolysogeny.” The PCR scanning analysis of other BoNTX/C1 and D phages based on the c-st sequence further revealed that BoNTX phages comprise a divergent phage family, probably generated by exchanging genomic segments among BoNTX phages and their relatives.

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