Bacterial genes that are involved in resistance against phages are commonly clustered in specific genomic locations, forming genomic defence islands. One such gene is pglZ, which encodes a phosphatase that has previously been implicated in a unique phage resistance phenotype in Streptomyces coelicolor called phage growth limitation (Pgl). Although the molecular mechanism of Pgl-mediated resistance is unknown, S. coelicolor strains carrying the Pgl system are susceptible to the first cycle of infection by the ΦC31 phage but are resistant to the progeny phages that are produced during this first cycle of infection.
To investigate the distribution of pglZ, Goldfarb, Sberro, et al. analysed more than 1,500 bacterial and archaeal genomes and detected genes homologous to pglZ in approximately 10% of the genomes, with a wide phylogenetic distribution. In most cases, pglZ is part of a six-gene cassette that the authors termed BREX; in addition to pglZ, it includes pglX, which encodes a methyltransferase that has also been associated with the Pgl phenotype, and brxABCL, which encodes an RNA-binding anti-termination protein (BrxA), a protein of unknown function (BrxB), an ATP-binding protein (BrxC) and a protease (BrxL). To test whether BREX is involved in the defence against phages, the authors cloned the complete BREX system from Bacillus cereus into Bacillus subtilis, which lacks an endogenous BREX system. Notably, BREX expression in B. subtilis strains conferred resistance against infection with both lytic and lysogenic phages.
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