ABSTRACTThe dairy industry uses the mesophilic, Gram-positive, lactic acid bacterium (LAB)Lactococcus lactisto produce an array of fermented milk products. Milk fermentation processes are susceptible to contamination by virulent phages, but a plethora of phage control strategies are available. One of the most efficient is to use LAB strains carrying phage resistance systems such as abortive infection (Abi) mechanisms. Yet, the mode of action of most Abi systems remains poorly documented. Here, we shed further light on the antiviral activity of the lactococcal AbiT system. Twenty-eight AbiT-resistant phage mutants derived from the wild-type AbiT-sensitive lactococcal phages p2, bIL170, and P008 were isolated and characterized. Comparative genomic analyses identified three different genes that were mutated in these virulent AbiT-insensitive phage derivatives:e14(bIL170 [e14bIL170]),orf41(P008 [orf41P008]), andorf6(p2 [orf6p2] and P008 [orf6P008]). The genese14bIL170andorf41P008are part of the early-expressed genomic region, but bioinformatic analyses did not identify their putative function.orf6is found in the phage morphogenesis module. Antibodies were raised against purified recombinant ORF6, and immunoelectron microscopy revealed that it is the major capsid protein (MCP). Coexpression inL. lactisof ORF6p2and ORF5p2, a protease, led to the formation of procapsids. To our knowledge, AbiT is the first Abi system involving distinct phage genes.
HK97 gp74 Possesses an α-Helical Insertion in the ββα Fold That Affects Its Metal Binding, cos Site Digestion, and In Vivo Activities
