Three classes of plasmid (47-63 kb) carry the type B neurotoxin gene cluster of group II Clostridium botulinum.
Bottom Line: Unexpectedly, no neurotoxin genes were found on the chromosome.This apparent constraint on neurotoxin gene transfer to the chromosome stands in marked contrast to Group I C. botulinum, in which neurotoxin gene clusters are routinely found in both locations.A plasmid toxin-antitoxin system pemI gene located close to the neurotoxin gene cluster and conserved in each type B4 plasmid class may be important in understanding the mechanism which regulates this unique and unexpected bias toward plasmid-borne neurotoxin genes in Group II C. botulinum type B4.
Affiliation: Gut Health and Food Safety, Institute of Food Research, Norwich Research Park, Norwich, United Kingdom firstname.lastname@example.org.Show MeSH
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Mentions: Complete plasmid sequences were compared using ACT (fig. 2). This revealed two different levels of relatedness between the plasmids. Evidence of relatively minor variation was small insertion/deletion events involving up to four CDSs, plus more scattered sequence differences, mostly single nucleotide polymorphisms (data not shown), presumably indicative of genetic drift. The parameters of the ACT analysis depicted in figure 2 are set so that sequence identity of approximately 90% or more appears as a solid red block connecting each pairwise comparison, so at the degree of magnification used in figure 2, the latter form of genetic variation is masked. Allowing for up to 10% sequence differences, the plasmids could be organized into three main classes; class 1, represented by pCB17B of Eklund 17B comprised seven members, each of approximately 48 kb; class 2, represented by pCDC 3875 comprised three members of sizes between approximately 58–63 kb; class 3 is represented by its only member, pIFR 05/025 (60 kb), which appeared to be a hybrid version of the two other classes. Allowing for the genetic drift already discussed, the regions of similarity between each plasmid class are quite marked, being defined by large blocks of sequence rather than by regions of intermittent homology (fig. 2A). Figures 2B and C demonstrate the high degree of relatedness between members within a single class, a fact which facilitated identification of the inserted/deleted CDSs. DNA sequences were used to generate phylogenetic trees; of the complete plasmid (fig. 3A), the neurotoxin gene cluster (fig. 3B) and, together with those also available in GenBank, the type B4 neurotoxin gene (fig. 3C). Three distinct groups of neurotoxin gene cluster and neurotoxin genes were identified (figs. 3B and C). As expected for Group II C. botulinum type B, all neurotoxin genes were of subtype B4; their DNA sequence variation fell within the range for subtype B4 (<48 nt) observed in a previous study of C. botulinum neurotoxin gene diversity (Hill et al. 2007).Fig. 2.—
Affiliation: Gut Health and Food Safety, Institute of Food Research, Norwich Research Park, Norwich, United Kingdom email@example.com.