Islander: a database of integrative islands in prokaryotic genomes, the associated integrases and their DNA site specificities

Prokaryotic chromosomes often contain islands, such as temperate phages or pathogenicity islands, delivered by site-specific integrases. Integration usually occurs within a tRNA or tmRNA gene, splitting the gene, yet sequences within the island restore the disrupted gene. The regenerated RNA gene and the displaced fragment of that gene thus mark the endpoints of the island. We applied this principle to search for islands in genomic DNA sequences. Our algorithm generates a list of tRNA and tmRNA genes, uses each as the query for a BLAST search of the starting DNA and removes unlikely hits through a series of filters. A search for islands in 106 whole bacterial genomes produced 143 candidates, with the search itself providing an estimate of three false candidates among these. Preliminary phylogenetic analysis of the associated integrases reduced this set to 89 cases of independently evolved site specificity, which showed strong bias for the tmRNA gene. The website Islander (hftp://www.indiana.edu/similar toislander) presents the candidate islands in GenBank-style files and correlates integrase phylogeny with site specificity.