Plasmid-mediated horizontal gene mobilisation: Insights from two lactococcal conjugative plasmids

The distinct conjugation machineries encoded by plasmids pNP40 and pUC11B represent the most prevalent plasmid transfer systems among lactococcal strains. In the current study, we identified genetic determinants that underpin pNP40- and pUC11B-mediated, high-frequency mobilisation of other, non-conjugative plasmids. The mobilisation frequencies of the smaller, non-conjugative plasmids and the minimal sequences required for their mobilisation were determined, owing to the determination of the oriT sequences of both pNP40 and pUC11B, which allowed the identification of similar sequences in some of the non-conjugative plasmids that were shown to promote their mobilisation. Furthermore, the auxiliary gene mobC, two distinct functional homologues of which are present in several plasmids harboured by the pNP40- and pUC11B-carrying host strains, was observed to confer a high-frequency mobilisation phenotype. These findings provide mechanistic insights into how lactococcal conjugative plasmids achieve conjugation and promote mobilisation of non-conjugative plasmids. Ultimately, these insights would be harnessed to optimise conjugation and mobilisation strategies for the rapid and predictable development of robust and technologically improved strains.