“Pathogenicity Islands” are the Bacterial Genome Mobile Elements.
[The concept founded by Dr. Jorg Hacker and coauthors, (6)]
Pathogenicity factors are predominantly encoded by or associated with mobile genetic elements such as phages, plasmids, insertion elements, or transposons, and a large number of such determinants are located with PAI (Pathogenicity Islands). These elements pay a pivotal role in virulence of bacterial pathogens and are also essential for virulence in pathogens of animal and plants.
More frequently, PAI are included in bacterial genome and occupy relatively large genomic regions. They are in range of 10 to 200 kb.
PAI are present in the genome of bacterial virulent forms but absent from the genomes of non-virulent representative of the same species. PAI may carry one or more virulence genes. However genomic elements with characteristics similar to PAI but lacking virulence genes are referred to as “genomic or metabolic islands”.
PAI often differ from the core genome in their base composition that is expressed as a percentage of guanine and cytosine (G+C). The average G+C content of bacterial DNA can range from 25 to 75%. Usually the horizontally acquired PAI has the base composition of the donor species.
PAI frequently located adjacent to tRNA genes that apparently may serve as anchor points for insertion of foreign DNA acquired by horizontal gene transfer. After acquisition by horizontal gene transfer a DNA fragment that contains the genes can insert into recipients genome by recombination between the tRNA genes.
PAI are frequently associated with mobile genetic elements. They can be flanked by direct repeats (DR) that are presented as DNA sequences of 16 to 20 bases pairs (up to 130 bp). DR might have served as recognition sites for the integration of bacteriophages and for enzymes involved in excision of mobile genetic elements, thus contributing to the instability of a PAI flanked by a DR. Some PAI contain genes that are similar to integrase and resolvase genes of transposons. These mobile genetic elements can change their location within the chromosome, but transposons can also jump from a chromosomal location into a plasmid and vice versa. Insertion sequence elements presented in PAI can also result in the mobilization of large portions of DNA. Integrated plasmids, conjugated transposons, bacteriophages or parts of these elements can also be observed in PAI. Pathogenicity factors encoded by PAI are lost with a frequency that is higher than the normal rate of mutations. The genetic mechanism that allows the distribution of PAI by horizontal gene transfer, determines their intrinsic genetic instability. Several characteristic elements such as integrases, transposases, and insertion elements, have been identified that contribute to mobilization and as well as instability of PAI.
Modern knowledge about the features of PAI gives evidence that the genomes of prokaryotes have a highly diverse mosaic structure. Besides a core genome, which mostly demonstrates homogeneous G+C content and codon usage, there exists a flexible gene pool that is formed by mobile genetic elements.
Yurii V. Ezepchuk, Ph.D.
Professor of Biochemistry
Denver, Colorado, U.S.A.