- Pathogenicity island
Pathogenicity islands (PAIs) are a distinct class of
genomic islands which are acquired by horizontal transfer.
They are incorporated in the
genomeof pathogenic microorganismsbut are usually absent from those non-pathogenic organisms of the same or closely related species. They usually occupy relatively large genomic regions ranging from 10-200 kb and encode genes which contribute to the virulenceof the respective pathogen. Typical examples are adherence factors, toxins, iron uptake systems, invasion factors and secretion systems.
Pathogenicity islands are discrete genetic units flanked by direct repeats, insertion sequences or tRNA genes, which are sites for recombination into the DNA. Cryptic mobility genes may also be present, indicating the provenance as transduction.
An analogous genomic structure in
rhizobiais termed a symbiosis island.
Properties of PAIs
1. PAIs carry genes encoding one or more virulence factors:
adhesins, toxins, invasins, etc.
2. They are located on bacterial chromosome or may be a part of a plasmid
3. The G+C content of pathogenicity islands often differs from that of the rest of the genome [Hacker J, Kaper JB., Pathogenicity islands and the evolution of microbes. "Annu Rev Microbiol". 2000 ;54:641-79 http://www.ncbi.nlm.nih.gov/pubmed/11018140?dopt=Abstract ]
4. They are flanked by direct repeats: The sequence of bases at two ends are the same.
5. PAIs are associated with tRNA genes, which target sites for the integration of DNA
6. PAIs carry functional genes, e.g. integrase, transposase, or part of insertion sequences.
7. Represent unstable DNA regions. May move from one tRNA locus to another or be deleted.
8. Site specific integration of pathogenicity islands is mediated by an Integrase recombinase
*The UPEC P fimbriae island contains virulence factors such as haemolysin, pili and cytotoxic necrosing factor
Yersinia pestisHigh Pathogenicity Island I has genes regulating iron uptake and storage
Facts about PAIs
One species of bacteria may have more than one PAI. (i.e. Salmonella has at least 5). PAIs are located mostly in gram-negative cells, but have been shown to appear in some gram-positive cells. They are located in pathogens that undergo gene transfer by plasmid, phage, or a conjugative transposon. They are transferred through
horizontal gene transfer.
* [http://www.jenner.ac.uk/BacBix3/BACdef.htm BAC definition of pathogenicity]
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