title: Evolution of Chromosomal Clostridium botulinum Type E Neurotoxin Gene Clusters: Evidence Provided by Their Rare Plasmid-Borne Counterparts.
abstract: Analysis of more than 150 Clostridium botulinum Group II type E genomes identified a small fraction (6%) where neurotoxin-encoding genes were located on plasmids. Seven closely related (134-144 kb) neurotoxigenic plasmids of subtypes E1, E3, and E10 were characterized; all carried genes associated with plasmid mobility via conjugation. Each plasmid contained the same 24-kb neurotoxin cluster cassette (six neurotoxin cluster and six flanking genes) that had split a helicase gene, rather than the more common chromosomal rarA. The neurotoxin cluster cassettes had evolved as separate genetic units which had either exited their chromosomal rarA locus in a series of parallel events, inserting into the plasmid-borne helicase gene, or vice versa. A single intact version of the helicase gene was discovered on a nonneurotoxigenic form of this plasmid. The observed low frequency for the plasmid location may reflect one or more of the following: 1) Less efficient recombination mechanism for the helicase gene target, 2) lack of suitable target plasmids, and 3) loss of neurotoxigenic plasmids. Type E1 and E10 plasmids possessed a Clustered Regularly Interspaced Short Palindromic Repeats locus with spacers that recognized C. botulinum Group II plasmids, but not C. botulinum Group I plasmids, demonstrating their long-term separation. Clostridium botulinum Group II type E strains also carry nonneurotoxigenic plasmids closely related to C. botulinum Group II types B and F plasmids. Here, the absence of neurotoxin cassettes may be because recombination requires both a specific mechanism and specific target sequence, which are rarely found together.
authors.affiliation: Gut Health and Food Safety, Institute of Food Research, Norwich Research Park, Norwich, United Kingdom andrew.carter@ifr.ac.uk.
authors.forename: Andrew T
authors.initials: AT
authors.lastname: Carter
authors.affiliation: Bureau of Microbial Hazards, Health Products and Food Branch, Health Canada, Ottawa, ON, Canada.
authors.forename: John W
authors.initials: JW
authors.lastname: Austin
authors.affiliation: National Microbiology Laboratory, Public Health Agency of Canada, Winnipeg, MB, Canada.
authors.forename: Kelly A
authors.initials: KA
authors.lastname: Weedmark
authors.affiliation: Gut Health and Food Safety, Institute of Food Research, Norwich Research Park, Norwich, United Kingdom.
authors.forename: Michael W
authors.initials: MW
authors.lastname: Peck
mesh_terms: D001905:Botulinum Toxins / Q000235:genetics*; D001906:Botulism / Q000235:genetics* / Q000382:microbiology; D047970:Clostridium botulinum type E / Q000235:genetics* / Q000472:pathogenicity; D016680:Genome, Bacterial; D006801:Humans; D005810:Multigene Family; D010802:Phylogeny; D010957:Plasmids / Q000235:genetics
chemical_list: D001905:Botulinum Toxins; C096326:botulinum toxin type E
keywords: CRISPR; botulism; genome evolution; horizontal gene transfer