The CTXφ bacteriophage is a
filamentous bacteriophage. It is a
positive-strand
DNA virus with
single-stranded DNA (ssDNA).
CTXφ infects some
strains of ''
Vibrio cholerae
''Vibrio cholerae'' is a species of Gram-negative bacteria, Gram-negative, Facultative anaerobic organism, facultative anaerobe and Vibrio, comma-shaped bacteria. The bacteria naturally live in Brackish water, brackish or saltwater where they att ...
'', the
bacterium
Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among the ...
that causes
cholera
Cholera () is an infection of the small intestine by some Strain (biology), strains of the Bacteria, bacterium ''Vibrio cholerae''. Symptoms may range from none, to mild, to severe. The classic symptom is large amounts of watery diarrhea last ...
. It carries the genes for
cholera toxin (CTX), which makes cholera especially
virulent. It can carry genes from one ''V. cholerae'' strain to another.
Genetic material
CTXφ is generally present and integrated into the
genome
A genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding genes, other functional regions of the genome such as ...
of the ''V. cholerae'' bacterium, and more rarely in a
virion
A virion (plural, ''viria'' or ''virions'') is an inert virus particle capable of invading a Cell (biology), cell. Upon entering the cell, the virion disassembles and the genetic material from the virus takes control of the cell infrastructure, t ...
from outside the bacterium. While integrated into the bacterial genome, CTX
prophage
A prophage is a bacteriophage (often shortened to "phage") genome that is integrated into the circular bacterial chromosome or exists as an extrachromosomal plasmid within the bacterial cell (biology), cell. Integration of prophages into the bacte ...
s are found on each of the two chromosomes (in the O1
serogroup of ''V. cholerae'') or arranged in
tandem
Tandem, or in tandem, is an arrangement in which two or more animals, machines, or people are lined up one behind another, all facing in the same direction. ''Tandem'' can also be used more generally to refer to any group of persons or objects w ...
on the larger chromosome (in the
El Tor biotype of ''V. cholerae'').
The genome of CTXφ is 7
kb long and consists of two regions. The core region, which is approximately 4,6 kb long, contains 5 genes that are associated with phage assembly and morphogenisis (''psh, cep, pIIICTX, ace, zot)'' and two non structural genes. The two non structural genes encoded in the core region of the phage are ''ctxA'' and ''ctxB'' which are the genes encoding for the Cholera Toxin (CT). The RS2 region, which measures 2,4 kb, contains genes that control the replication, regulation, and integration of CTXφ. The three genes encoded in the RS2 region are ''rstR'', ''rstA'' and ''rstB.''
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 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 is the TolQRA membrane protein structure.
These requirements for phage membrane binding are quite similar to those of the
Ff phages, which infects the bacterium ''
Escherichia coli
''Escherichia coli'' ( )Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. is a gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus '' Escherichia'' that is commonly fo ...
'' 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
A plasmid is a small, extrachromosomal DNA molecule within a cell that is physically separated from chromosomal DNA and can replicate independently. They are most commonly found as small circular, double-stranded DNA molecules in bacteria and ...
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