Clostridium perfringens type A-E toxin plasmids

Abstract

Clostridium perfringens relies upon plasmid-encoded toxin genes to cause intestinal infections. These toxin genes are associated with insertion sequences that may facilitate their mobilization and transfer, giving rise to new toxin plasmids with common backbones. Most toxin plasmids carry a transfer of clostridial plasmids locus mediating conjugation, which likely explains the presence of similar toxin plasmids in otherwise unrelated C. perfringens strains. The association of many toxin genes with insertion sequences and conjugative plasmids provides virulence flexibility when causing intestinal infections. However, incompatibility issues apparently limit the number of toxin plasmids maintained by a single cell.

Keywords: Clostridia; Conjugation; Gastrointestinal disease; Plasmid evolution; Plasmid-encoded toxin.

Figure 1. Comparison of C. perfringens plasmid diversity and organization

The sequenced plasmids of C. perfringens are represented graphically. Panel A shows an aligned comparison of tcp-negative plasmids pCP13 (AP003515.1 [72]) and BEC-encoding plasmid pCP-OS1 (AP013033 [71]) demonstrating significant homology between these two plasmids. Panel B shows plasmids with a pCP13-like backbone that harbors the tcp locus. Depicted are plasmids: pCW3 (DQ366035 [129]); pJIR3844 (JN689217 [61]); pJIR3535 (JN689219 [61]); pCP8533etx (AB444205 [70]); pCPF5603 (AB236337 [36)]; pCPPB-1 (AB604032 [44]); pCPF4969 (AB236337 [36]). The conserved region of these C. perfringens plasmids is shown at the top of Panel B, with variable regions displayed graphically below. Arrows represent ORFs, and are colored as follows in Panel B: red arrows – tcp conjugation locus; dark blue arrows – conserved ORFs; yellow arrows – plasmid replication region; light blue arrows – ORFs unique to each plasmid; fuchsia arrows – tetracycline resistance genes; green arrows – cpb2; purple arrow – netB; pink arrow – etx; gray arrows – cpe; dark gray arrows – iap/ibp. Asterisks designate toxin genes. Modified with permission from [122].

Figure 2. The genetic organization of the pCW3 tcp locus

Proteins encoded by the various genes are stated below the arrows. The patterned arrows indicate Tcp proteins involved in pCW3 conjugative transfer, black arrows depict non-essential Tcp proteins and the grey arrow the IntP protein, which is currently uncharacterized. Proteins with sequence, functional or structural similarity from the paradigm VirB/D system from the Ti plasmid from the Gram-negative Agrobacterium tumefaciciens, the conjugation region from Tn916 and the broad-host range plasmid pIP501 from the Gram-positive Streptococcus agalactiae are represented by arrows with similar patterns. Each of these conjugation regions has homologs of the putative coupling proteins, VirD4 and TcpA, VirB8-like proteins, VirB4-like ATPases, VirB1-like lytic transglycolases and VirB6-like proteins. Based on data from [128, 129, 135].

Figure 3. Model for evolution of the C. perfringens toxin plasmids

A model of evolution for sequenced and characterized C. perfringens toxin plasmids is shown. A) A pCP13-like plasmid acquires the becAB locus (pCP-OS1 and pCP-TS1 [71]) or the tcp locus (pCP13 [72]). B) The further evolution of pCW3 [129], pJIR3535 [61], pCP8533etx [70], pCPF5603 [36], pCPPB-1 [44], and pCPF4969 [36] are diagrammed. See section 3.3 for a discussion of evolution of these plasmids. Note that important plasmid regions are color coded. Modified with permission from [122].