From dairy products to molecular mechanisms: characterization of Lactococcus lactis phage proteins involved in Homologous Recombination and Abortive Infection systems

Virulent phages of the 936 and P335 group are predominant in nfections of Lactococcus lactis, a Gram-positive bacterium widely used in the dairy industry. Among the strategies adopted by L. lactis against virus infection, pivotal roles are played by abortive infection mechanisms (Abi), but little is known about their molecular details. With regard to the AbiK system, four non-similar phage proteins, involved in the sensibility to AbiK and in homologous recombination replication, were identified and named Sak (sensitive to AbiK). Here is presented the biochemical and structural characterization of two of them: Sak from phage ul36, an homologue of the eukaryotic protein RAD52 with a clear DNA Single-Strand Annealing function (SSA proteins) and Sak3 from phage p2, the reference protein for the third group of identified sak genes that do not show significant sequence similarity to SSAPs. In addition, we report the three dimensional structure and functional characterization of a third protein, a phage p2 single-strand binding protein (SSB), which is a Sak3 partner in homologous recombination. With our investigation, we could establish, experimentally, a relationship between phage protein mediated DNA processing and L.lactis antiviral AbiK system.