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 lambda 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.(ABSTRACT TRUNCATED AT 250 WORDS)

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T-even coliphages have 5-hydroxymethylcytosine in their DNA instead of cylosine. In some T4 mutants, the replicated DNA contains cytosine, but then no late gene products are made. We show that the inability to make late gene products with cytosine-containing T4 DNA is due to a T4 gene product. This gene product, while probably nonessential under normal conditions, interacts with an essential part of the transcription apparatus. Mutations in this gene allow viable T4 particles to be made whose DNA has been substituted almost 100% with cytosine.Studies of the interaction between the T-even coliphages T2, T4, and T6 and their host have been central to the development of modern biology. One reason for their importance is that the DNA of these bacteriophages contains an unusual base, 5-hydroxymethylcytosine, instead of the usual cytosine. It was the appearance, after infection, of enzymes responsible for synthesizing this base that led to the concept of virus-induced functions (1). The presence of hydroxymethylcytosine protects the phage DNA against the normal host restriction systems (rK mK or rB mB) but opens it to attack by other restriction systems coded by the rgl genes (2). In turn, the existence of virus-coded glucosyltransferases, which put glucose on the hydroxymethyl group of hydroxymethylcytosine, makes them immune to the rgl systems (3). Presumably because they have hydroxymethylcytosine, it is possible for the T-even coliphages to code for deoxyribonucleases that are specific for cytosine-containing DNA and therefore to degrade host DNA in the presence of bacteriophage DNA. Host DNA degradation is a nonessential source of nucleotides for bacteriophage DNA replication.By mutating the functions required for the synthesis of hydroxymethylcytosine and the functions required to degrade cytosine-containing DNA, it should be possible to make T4 with cytosine in its DNA instead of hydroxymethylcytosine. In fact, it has been known for some time that with the right combination of mutations, T4 DNA that contains cytosine is replicated. The mutations required are apparent from the scheme shown in Fig. 1. In essence, T4 prevents cytosine from entering its DNA by producing a dCTPase (4) (gene 56) which dephosphorylates dCTP and dCDP (5, 6) and furnishes substrate (dCMP) for a hydroxymethylase (gene 42) (7). The substrate, dCMP, is also furnished by the breakdown of host DNA, which requires at least T4 endodeoxyribonuclease II (gene denA) (8-10) and an exonuclease activity (genes 46 and 47) (11,12). Thus, a dCTPase-mutant should have some cytosine in its replicated DNA but should have hydroxymethylcytosine as well, since dCMP substrate for hydroxymethylase is being furnished by host breakdown and, possibly, from dCDP by the reversal of the host dCMP kinase reaction. Even though it has some hydroxymethylcytosine, this DNA is degraded by the enzymes responsible for degrading host DNA because of the presence of cytosine. These enzymes are the same ones required for host DNA breakdown, except that T4 endodeoxyribo...

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