BREX systems (bacteriophage exclusion) provide immunity against bacteriophage. Part of a type 1 BREX system which protects against dsDNA phage. This system allows phage adsorption but prevents phage DNA replication, without degradation of the phage DNA. Methylation of bacterial DNA by this protein guides self/non-self discrimination. When the brxA-brxB-brxC-pglX-pglZ-brxL genes are transformed into a susceptible E.coli strain (BW25113) they confer very high resistance to infection by bacteriophage VR7 and VpaE1, about 100-fold protection against lambda, T5 and T7 (probably with a mutated 0.3 gene) and no protection against RNA phage Qbeta, ssDNA phage M13 or dSDNA phage T4 and VR5. Glycosylated phage DNA is not susceptible to BREX. The BREX system does not confer resistance to lysogenic lambda phage, i.e. prophage that are integrated into the chromosomal DNA and then induced to form phage. Methylates the adenine in the fifth position of the hexamer 5'-GGTAAG-3' in genomic DNA; methylates the same sequence in the few phage that escape the BREX system. Methylated phage are now resistant to BREX, showing immunity is provided by an epigenetic modification. Expression of this protein alone has no effect on phage infection, does not lead to methylated DNA and mildly inhibits growth. a 2'-deoxyadenosine in DNA + S-adenosyl-L-methionine = an N(6)-methyl-2'-deoxyadenosine in DNA + H(+) + S-adenosyl-L-homocysteine Methyltransferase activity is partially inhibited (20% reduction) by phage T7 protein OCR (AC P03775, gene 0.3). Viability of cells overexpressing OCR is unaffected. Interacts with phage T7 protein Ocr (AC P03775, gene 0.3) during an infection and when the protein is expressed from a plasmid. Transcribed at slowly increasing levels as cells progress from lag to exponential to stationary phase. No methylation of 5'-GGTAAG-3' in chromosomal DNA, BREX no longer confers phage resistance. Belongs to the methyltransferase superfamily. PglX adenine methyltransferase family.