PHENOTYPIC AND GENETIC-CHARACTERIZATION OF THE BACTERIOPHAGE ABORTIVEINFECTION MECHANISM ABIK FROM LACTOCOCCUS-LACTIS

The natural plasmid pSRQ800 isolated from Lactococcus lactis subsp. la ctis W1 conferred strong phage resistance against small isometric phag es of the 936 and P335 species when introduced into phage-sensitive L. lactis strains. It had very limited effect on prolate phages of the c 2 species. The phage resistance mechanism encoded on pSRQ800 is a temp erature-sensitive abortive infection system (Abi), Plasmid pSRQ800 was mapped, and the Abi genetic determinant was localized on a 4.5-kb Eco RI fragment. Cloning and sequencing of the 4.5-kb fragment allowed the identification of two large open reading frames. Deletion mutants sho wed that only orf1 was needed to produce the Abi phenotype. orf1 (rena med abiK) coded for a predicted protein of 599 amino acids (AbiK) with an estimated molecular size of 71.4 kDa and a pi of 7.98, DNA and pro tein sequence alignment programs found no significant homology with da tabases. However, a database query based on amino acid composition sug gested that AbiK might be in the same protein family as AbiA, No phage DNA replication nor phage structural protein production was detected in infected AbiK(+) L. lactis cells. This system is believed to act at or prior to phage DNA replication, When cloned into a high-copy vecto r, AbiK efficiency increased 100-fold, AbiK provides another powerful tool that can be useful in controlling phages during lactococcal ferme ntations.