Mobilizable genomic islands: going mobile with oriT mimicry

Many bacterial chromosomes contain genomic islands, large DNA segments that became incorporated into the chromosome following their horizontal transmission. However, the mechanisms that mediate the lateral transfer of genomic islands are for the most part unknown. In this issue of Molecular Microbiology, Daccord et al. describe a new type of transmissible genomic island that can be mobilized by co-resident integrating conjugative elements (ICEs). These 'mobilizable genomic islands' (MGIs) require many ICE-encoded factors for their transmission, including transcription activators to induce MGI excision, the relaxase to initiate transfer at the MGI oriT and the conjugation machinery to transport MGI DNA to recipient cells. However, MGIs encode their own integrases, which enable their recombination with the chromosome of a new host, as well as a variety of other genes that do not have functions related to mobility. The MGI oriT, which resembles the ICE oriT, can also serve as a site for initiation of ICE-mediated conjugative transfer of large regions of chromosomal DNA. Overall, these findings raise the possibility that mobilization of chromosomal DNA from cyptic oriTs within genomic islands or elsewhere on the chromosome could be more common place than has been previously appreciated.