Phage‐exclusion enzymes: a bonanza of biochemical and cell biology reagents?

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Abstract
Many parasitic DNA elements including prophages and plasmids synthesize proteins that kill the cell after infection by other phages, thereby blocking the multiplication of the infecting phages and their spread to other nearby cells. The only known function of these proteins is to exclude the infecting phage, and therefore to protect their hosts, and thereby the DNA elements themselves, against phage contagion. Many of these exclusions have been studied extensively and some have long been used in molecular genetics, but their molecular basis was unknown. The most famous of the phage exclusions are those caused by the Rex proteins of λ prophage. The Rex exclusions are still not completely understood, but recent evidence has begun to lead to more specific models for their action. One of the Rex proteins, RexA, may be activated by a DNA‐protein complex, perhaps a recombination or replication intermediate, produced after phage infection. In the activated state, RexA may activate RexB, which has been proposed to be a membrane ion channel that allows the passage of monovalent cations, destroying the cellular membrane potential, and killing the cell. We now understand two other phage exclusions at the molecular level which use strategies that are remarkably similar to each other. The parasitic DNA elements responsible for the exclusions both constitutively synthesize enzymes that are inactive as synthesized by the DNA element but are activated after phage infection by a short peptide determinant encoded by the infecting phage. In the activated state, the enzymes cleave evolutionarily conserved components of the translation apparatus, in one case EF‐Tu, and in the other case tRNALys. Translation is blocked and development of the phage is arrested. A myriad of different phage‐exclusion systems are known to exist and many of these may also be specific for highly conserved cellular components, furnishing generally useful enzymes for biochemical and biomedical research.