Symbiosis Islands

Genomic islands are regions of the bacterial genome (usually the chromosome) that appear to have been acquired through horizontal gene transfer. Genomic islands can harbor genes for pathogenicity, metabolism, antibiotic resistance and symbiosis. Rhizobia are soil bacteria that can establish nitrogen-fixing symbioses with leguminous plants. In Bradyrhizobium and Mesorhizobium spp., symbiosis genes are commonly located within genomic islands termed “symbiosis islands”. Symbiosis islands in Mesorhizobium are members of a class of mobile genetic elements called integrative and conjugative elements (ICEs) that horizontally transfer to other mesorhizobia via conjugation. Symbiosis ICEs (ICESyms) confer symbiotic ability to the recipient bacterium and carry a wide range of other genes that may contribute to the plant-symbiont interaction. Individual ICESyms integrate at a specific tRNA gene or other housekeeping gene, and their integration, excision, and transfer are controlled by ICE-encoded genes whose expression is under complex regulation involving quorum sensing. In agriculture, acquisition of an ICESym from an inoculant strain can convert saprophytic soil bacteria into novel symbionts that then outcompete the inoculant for nodulation of the legume, although not all the newly evolved symbionts effectively fix nitrogen. ICESyms likely evolved from a common ICE ancestor, which itself evolved from a large family of ICEs distributed throughout the proteobacteria.