The DNA of bacteria is dynamic, other than the core genome; it’s composed of mobile elements, for instance, transposons, a genetic island that can be transferred through a horizontal gene system to the other strains or host.
Some pathogenic bacteria have virulence factors to initiate the pathogenesis inside the host, emphasis has been given on a molecular basis that encodes virulence factors.
In bacterial chromosomes, extended isolated genes are clustered together in a pathogenicity island or virulence island, generally surrounded by mobile elements like transposon which permits the movement in the chromosome or to the other bacteria.
This whole genetic unit comes in the action by the surrounding stimulus (such as pH, contact with the host surface, heat) by which it is coordinated and expressed.
For e.g., Salmonella has SPI-1 island that encodes 25 genes for the type III secretion system, which permits access into the non-phagocytic cells.
Pathogenicity Island may be defined as “a set of genes which encodes for various virulence factors and it acquires large genetic regions of the chromosome or the plasmids, expression of genes are coordinated together”.
General Attributes of the PAI
Carry more than one gene for virulence, when ‘vir’ gene is lacking, it is referred to as genomic or metabolism island.
Usually, PAI is present in virulent strains of bacteria or the genome of the pathogenic bacteria.
The majority of PIA is a range of 10-200kb.
Base composition i.e., G+C content is different from than core genome; also shows different codon usage which is preferred by organisms.
Virulence genes are present.
PAI is inserted alongside the tRNA genes (3’ next to end) is usually seen in gram –ve bacteria; also encodes for SRS ( small regulatory sequence ) Rnase.
It is related to mobile genetic elements; DR repeat up to 9-131 bp pair length is present which is flanked, cryptic or functional integrase or transposon, IS elements.
Genetic instability, if transposon is present.
Why insertion of PAI is adjoined near the tRNA genes?
They can use the promoter region to get transcribed continuously.
The tRNA genes are always transcriptionally active, if they are on regulation; PAI will also be on the regulator.
The tRNA genes are the hotspots for recombination.
The tRNA genes, nucleotide sequence are highly conserved even among distantly related spp., which is used as integration sites, and may elevate the putative host range of mobile genetic elements.
The tRNA genes exhibit symmetrical sequence, which facilitates binding of protein, therefore recombination would occur efficiently.
The integration seems to occur preferentially into no- essentially tRNA genes; or those tRNA that has more copies of chromosomes.
Functions of PAI
They encode for colonization, invasion, toxin production which damage the tissue other than that virulence factors encoded by PAI are as follows: