Pathogenicity island

Pathogenicity islands (PAIs) are a distinct class of genomic islands which are acquired by horizontal transfer.

They are incorporated in the genome of pathogenic microorganisms but 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 virulence of the respective pathogen. Typical examples are adherence factors, toxins, iron uptake systems, invasion factors and secretion systems.

An analogous genomic structure in rhizobia is 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. They are high in Guanine + Cytosine content, meaning more G+C base pairs than A+T.

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.

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.