The CTXφ bacteriophage is a filamentous bacteriophage that contains the genetic material needed by the Vibrio cholerae bacterium for the production of cholera toxin, or CT. CTXφ is a positive virus with single-stranded DNA (ssDNA).
CTXφ is generally present as an integrated section of the genome of the V. cholerae bacterium, and more rarely in a virion form outside the bacterium. While integrated into the bacterial genome, the CTX prophages are found on each of the two chromosomes (O1 serogroup of V. cholerae) or arranged in tandem on the larger chromosome of bacterium (El Tor biotype of V. cholerae). The genome of CTXφ is 6.9 kb long and consists of two regions. The core region contains genes that code for the CT toxin itself as well as some proteins that are presumed to form the outside coat of the virus. The RS2 region contains the genes that control the replication, regulation, and integration of CTXφ.
Insertion, replication and release from host cell
The insertion of the CTXφ genetic material into the V. cholerae bacterium is mediated by two receptors. The first of these is the Toxin-Coregulated Pilus (TCP), which also aids the bacterium in adhering to the intestinal cell wall. The TCP is thought to bind to the minor CTXφ coat protein OrfU. The second of the two receptors is the TolQRA membrane protein structure.
These requirements for phage membrane binding are quite similar to those of the Ff bacteriophage, which infects the bacterium Escherichia coli and requires the F pilus along with the TolQRA structure. In E. coli, the TolQRA acts to translocate the Ff phage into the periplasmic space, where a possible membrane fusion event leads to the insertion of the Ff genome into the E. coli cytoplasm. A similar mechanism is suspected in the injection of the CTXφ genetic material into the V. cholerae cell, though further research is needed to confirm this.
After ssDNA insertion into the V. cholerae cytoplasm, a complementary strand of DNA is formed in order to create the plasmid form of the viral genome, pCTX. pCTX can undergo DNA replication to create new ssDNA genomes and/or be incorporated into the bacterial genome as a prophage. Because this prophage can be present in tandem in some biotypes of V. cholerae, horizontal and vertical transmission of the genetic material from CTXφ are not mutually exclusive.
After the production of the proteins and genomic material necessary to create new virion forms of the bacteriophage, the proteins assemble at a membranous protein complex, the EpsD secretin. Once the new ssDNA genome in inside the assembled proteins, the CTXφ virion detaches from the EpsD and is free to infect other bacteria.
Recent research suggests that there are at least two toxins other than CT produced from the genomic material contained within the CTXφ genome. The first of these is accessory cholera enterotoxin (Ace). Ace is currently thought to be a minor coat protein of virion stage CTXφ, though the process by which the toxin could be released from the protein coat has not yet been identified. The second non-CT toxin encoded within the CTXφ genome is zonula occludens toxin (Zot). Zot, though absolutely essential for the production of the CTXφ virion, is not actually present in the phage particle. The part that Ace and Zot play in the virulence of cholera is still quite unclear.
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