The plasticity of bacterial genomes enables the rapid adaptation to the changing environmental conditions. Horizontal gene transfer (HGT), by which bacteria can acquire and disseminate many beneficial traits, is mainly responsible for this plasticity. Conjugative elements such as plasmids, some genomic islands (GIs) and transposons have a key role in HGT since their mobility can lead to the rapid acquisition of virulence, pathogenicity or resistance factors and/or complete metabolic pathways of bacteria. Recent investigations of bacterial genomes put the mobile GIs in the spotlight of scientific interest. Mobile GIs are classified into two groups: integrative/conjugative elements (ICEs) and mobilizable genomic islands (MGIs). Unlike ICEs, MGIs are not self-transferable, so they require helper elements (ICEs or conjugative plasmids) that can provide the missing conjugative functions. GIs are stable part of the bacterial chromosome. They can not maintain extrachromosomally, but can be excised by site-specific recombination, transferred to other bacteria by conjugation and integrated into the chromosome of the recipient cell. These maintenance and transfer functions are encoded by the “backbone” of GIs, while another group of genes coding for antibiotic resistance, pathogenicity, catabolic pathways etc. confers adaptive functions to the host. The structure of mobile GIs shows remarkable flexibility: they evolve by acquisition, deletion and exchange of genes or gene clusters via homologous and/or site-specific recombination or transposition. One of the most studied MGIs is the Salmonella genomic island 1 (SGI1), which contains several antibiotic resistance genes embedded in the complex In104 integron segment. Prototype of SGI was detected in multiresistant Salmonella enterica serovar Typhimurium DT104 isolates, but its variants have also been identified in many other Salmonella serovars and in Proteus mirabilis isolates. Interestingly, SGI1 has never been found in natural Escherichia coli isolates even though it can easily be transferred into E. coli under laboratory conditions. SGI1 is a typical MGI, which is mobilized exclusively by the conjugative helper plasmids of IncA/C family. The first step of SGI1 transfer is the excision from the bacterial chromosome, which is carried out by the SGI1-encoded site-specific recombinase Int and Xis. The induction of excision and the transfer process also requires helper plasmid-encoded functions.
Beside the conjugation apparatus (T4SS) of IncA/C plasmids, SGI1 also exploits regulatory functions of the helper plasmid. The plasmid-encoded FlhDC-family master regulator controls all conjugation genes of the plasmid including the relaxase, a key factor in the transfer initiation step, the operons of pilus assembly and perhaps some other functions. The master regulator triggers the excision of SGI1, i.e. signals the presence of a helper plasmid for SGI1, thus SGI1 hijacks both the regulation and the conjugation apparatus of IncA/C plasmids for its horizontal transfer. On the contrary, only SGI1 encoded functions are required for the integration into the recipient chromosome. SGI1 represents a good example how MGIs ensure their vertical transmission in absence of a helper and how they can exploit their helpers for horizontal spread
