Conjugative transfer of the Lactococcus lactis chromosomal sex factor promotes dissemination of the Ll.LtrB group II intron

The L1.LtrB group II intron from the low-G+C gram-positive bacterium Lactococcus lactis was the first bacterial group II intron shown to splice and mobilize in vivo. This retroelement interrupts the relaxase gene (ltrB) of three L. lactis conjugative elements: plasmids pRS01 and pAH90 and the chromosomal sex factor. Conjugative transfer of a plasmid harboring a segment of the pRS01 conjugative plasmid including the L1.LtrB intron allows dissemination of L1.LtrB among L. lactis strains and lateral transfer of this retroelement from L. lactis to Enterococcusfaecalis. Here we report the dissemination of the L1.LtrB group II intron among L. lactis strains following conjugative transfer of the native chromosomally embedded L. lactis sex factor. We demonstrated that L1.LtrB dissemination is highly variable and often more efficient from this integrative and conjugative element than from an engineered conjugative plasmid. Cotransfer among L. lactis strains of both L1.LtrB-containing elements, the conjugative plasmid and the sex factor, was detected and shown to be synergistic. Moreover, following their concurrent transfer, both mobilizable elements supported the spread of their respective copies of the L1.LtrB intron. Our findings explain the unusually high efficiency of L1.LtrB mobility observed following conjugation of intron-containing plasmids.