Diversity of Integrative and Conjugative Elements of Streptococcus salivarius and Their Intra- and Interspecies Transfer.

Integrative and conjugative elements (ICEs) are widespread chromosomal mobile genetic elements which can transfer autonomously by conjugation in bacteria. Thirteen ICEs with a conjugation module closely related to that of ICE of were characterized in by whole-genome sequencing. Sequence comparison highlighted ICE evolution by shuffling of 3 different integration/excision modules (for integration in the 3' end of the , , or gene) with the conjugation module of the ICE subfamily. Sequence analyses also pointed out a recombination occurring at (likely mediated by the relaxase) as a mechanism of ICE evolution. Despite a similar organization in two operons including three conserved genes, the regulation modules show a high diversity (about 50% amino acid sequence divergence for the encoded regulators and presence of unrelated additional genes) with a probable impact on the regulation of ICE activity. Concerning the accessory genes, ICEs of the ICE subfamily appear particularly rich in restriction-modification systems and orphan methyltransferase genes. Other cargo genes that could confer a selective advantage to the cell hosting the ICE were identified, in particular, genes for bacteriocin synthesis and cadmium resistance. The functionality of 2 ICEs of was investigated. Autonomous conjugative transfer to other strains, to , and to was observed. The analysis of the ICE- border sequence in these transconjugants allowed the localization of the DNA cutting site of the ICE integrase. The ICE subfamily of ICEs appears to be widespread in streptococci and targets diverse chromosomal integration sites. These ICEs carry diverse cargo genes that can confer a selective advantage to the host strain. The maintenance of these mobile genetic elements likely relies in part on self-encoded restriction-modification systems. In this study, intra- and interspecies transfer was demonstrated for 2 ICEs of Closely related ICEs were also detected in other species ( and ), thus indicating that diffusion of ICE-related elements probably plays a significant role in horizontal gene transfer (HGT) occurring in the oral cavity but also in the digestive tract, where is present.