BREX (for Bacteriophage Exclusion) is a family of anti-phage defense systems. BREX systems are active against both lytic and lysogenic phages. They allow phage adsorption but block phage DNA replication, and are considered to be [RM](/defense-systems/rm)-like systems (1,2). BREX systems are found in around 10% of sequenced microbial genomes (1).
BREX (for Bacteriophage Exclusion) is a family of anti-phage defense systems. BREX systems are active against both lytic and lysogenic phages. They allow phage adsorption but block phage DNA replication, and are considered to be [RM](/defense-systems/rm)-like systems :ref{doi=10.15252/embj.201489455,10.1093/nar/gkaa290}. BREX systems are found in around 10% of sequenced microbial genomes :ref{doi=10.15252/embj.201489455}.
BREX systems can be divided into six subtypes, and are encoded by 4 to 8 genes, some of these genes being mandatory while others are subtype-specific (1).
BREX systems can be divided into six subtypes, and are encoded by 4 to 8 genes, some of these genes being mandatory while others are subtype-specific :ref{doi=10.15252/embj.201489455}.
## Molecular mechanism
*B. cereus* BREX Type 1 system was reported to methylate target motifs in the bacterial genome (1). The methylation activity of this system has been hypothesized to allow for self from non-self discrimination, as it is the case for Restriction-Modification ([RM)](/defense-systems/rm) systems.
*B. cereus* BREX Type 1 system was reported to methylate target motifs in the bacterial genome :ref{doi=10.15252/embj.201489455}. The methylation activity of this system has been hypothesized to allow for self from non-self discrimination, as it is the case for Restriction-Modification ([RM)](/defense-systems/rm) systems.
However, the mechanism through which BREX Type 1 systems defend against phages is distinct from RM systems, and does not seem to degrade phage nucleic acids (1).
However, the mechanism through which BREX Type 1 systems defend against phages is distinct from RM systems, and does not seem to degrade phage nucleic acids :ref{doi=10.15252/embj.201489455}.
To date, BREX molecular mechanism remains to be described.
## Example of genomic structure
The BREX system have been describe in a total of 6 subsystems.
There are 6 subsystems described for the BREX system.
BREX systems necessarily include the pglZ gene (encoding for a putative alkaline phosphatase), which is accompanied by either brxC or pglY. These two genes share only a distant homology but have been hypothesized to fulfill the same function among the different BREX subtypes (1).
BREX systems necessarily include the pglZ gene (encoding for a putative alkaline phosphatase), which is accompanied by either brxC or pglY. These two genes share only a distant homology but have been hypothesized to fulfill the same function among the different BREX subtypes :ref{doi=10.15252/embj.201489455}.
Goldfarb and colleagues reported a 6-gene cassette from *Bacillus cereus* as being the model for BREX Type 1. BREX Type 1 are the most widespread BREX systems, and present two core genes (pglZ and brxC). Four other genes are associated with BREX Type 1 : *pglX (*encoding for a putative methyltransferase), *brxA (*encoding an RNA-binding anti-termination protein)*, brxB (*unknown functio*n), brxC (*encoding for a protein with ATP-binding domain) and *brxL* (encoding for a putative protease) (1,2).
Goldfarb and colleagues reported a 6-gene cassette from *Bacillus cereus* as being the model for BREX Type 1. BREX Type 1 are the most widespread BREX systems, and present two core genes (pglZ and brxC). Four other genes are associated with BREX Type 1 : *pglX (*encoding for a putative methyltransferase), *brxA (*encoding an RNA-binding anti-termination protein)*, brxB (*unknown functio*n), brxC (*encoding for a protein with ATP-binding domain) and *brxL* (encoding for a putative protease) :ref{doi=10.15252/embj.201489455,10.1093/nar/gkaa290}.
Type 2 BREX systems include the system formerly known as Pgl, which is comprised of four genes (pglW, X, Y, and Z) (3), to which Goldfarb and colleagues found often associated two additional genes (brxD, and brxHI).
Type 2 BREX systems include the system formerly known as Pgl, which is comprised of four genes (pglW, X, Y, and Z) :ref{doi=10.1093/nar/gky1125}, to which :ref{doi=10.15252/embj.201489455} found often associated two additional genes (brxD, and brxHI).
Although 4 additional BREX subtypes have been proposed, BREX Type 1 and Type 2 remain the only ones to be experimentally validated. A detailed description of the other subtypes can be found in Goldfarb *et al*., 2015.
Although 4 additional BREX subtypes have been proposed, BREX Type 1 and Type 2 remain the only ones to be experimentally validated. A detailed description of the other subtypes can be found in :ref{doi=10.15252/embj.201489455}.
Here is some example found in the RefSeq database:
