The ongoing co-evolution and co-adaptation of bacteria and bacteriophages has led to the development of antiviral defense systems that are encoded in regions of the bacterial genome called defense islands [1]. One of the bacterial antiviral defense systems is BREX (Bacteriophage Exclusion). It is present in about 10 % of prokaryotic genomes. This system inhibits bacteriophage replication by an unknown mechanism of action [2].
Type 1 BREX system consists of a cassette of 6 protein-coding genes: brxA, brxB, brxC, pglZ, brxL, pglX. The pglX gene encodes the m6A DNA methyltransferase, which methylates specific sequences in the host genome to distinguish itself from foreign DNA [2]. Bacteriophages can evade BREX protection by encoding protein inhibitors. The Ocr protein encoded by the T7 bacteriophage binds specifically to the PglX protein in a way that mimics the double-stranded helix of DNA and blocks BREX protection [3]. In this study, we analyse the influence of PglX mutations on BREX activity and its inhibition.