Genomic Island

A Genomic island (GI) is part of a genome that has evidence of horizontal origins. The term is usually used in microbiology, especially with regard to bacteria. A GI can code for many functions, can be involved in symbiosis or pathogenesis, and may help an organism's adaptation. Many sub-classes of GIs exist that are based on the function that they confer. For example a GI associated with pathogenesis is often called a pathogenicity island (PAIs), while GIs that contain many antibiotic resistant genes are referred to as antibiotic resistance islands. The same GI can occur in distantly related species as a result of various types of lateral gene transfer (transformation, conjugation, transduction). This can be determined by base composition analysis, as well as phylogeny estimations.

Many type 3 secretion systems and type 4 secretion systems are located on regions of DNA called Genomic Islands.These "islands" are characterised by their large size(>10 Kb),their frequent association with tRNA-encoding genes and a different G+C content compared with the rest of the genome.Many genomic islands are flanked by repeat structures and carry fragments of other mobile elements such as phages and plasmids.Some genomic islands can excise themselves spontaneously from the chromosome and can be transferred to other suitable recipients.