The rexA and rexB genes of bacteriophage A. encode a two-component system that aborts lytic growth of bacterial viruses. Rex exclusion is characterized by termination of macromolecular synthesis, loss of active transport, the hydrolysis of ATP, and cell death. By analogy to colicins El and K, these results can be explained by depolarization of the cytoplasmic membrane. We have fractionated cells to determine the intracellular location of the RexB protein and made RexB-alkaline phosphatase fusions to analyze its membrane topology. The RexB protein appears to be a polytopic transmembrane protein. We suggest that RexB proteins form ion channels that, in response to lytic growth of bacteriophages, depolarize the cytoplasmic membrane. The Rex system requires a mechanism to prevent X itself from being excluded during lytic growth. We have determined that overexpression of RexB in A. lysogens prevents the exclusion of both T4 rll mutants and X ren mutants. We suspect that overexpression of RexB is the basis for preventing self-exclusion following the induction of a \ lysogen and that RexB overexpression is accomplished through transcriptional regulation.

Bacteriophage T4 possesses three self-splicing group I introns. Two of the three introns are mobile elements; the third, in the gene encoding a subunit of the phage nucleotide reductase (nrdB), is not mobile. Because intron mobility offers a reasonable explanation for the paradoxical occurrence of large intervening sequences in a space-efficient eubacterial phage, it is puzzling that the nrdB intron is not mobile like its compatriots. We have discovered a larger nrdB intron in a closely related phage, and we infer from comparative sequence data that the T4 intron is a deletion mutant derived from this larger intron. This larger nrdB intron encodes an open reading frame of 269 codons, which we have cloned and overexpressed. The overexpressed protein shows a dsDNA endonuclease activity specific for the intronless nrdB gene, typical of mobile introns. Thus, we believe that all three introns of T4 are or were mobile "infectious introns" and that they have entered into and been maintained in the phage population by virtue of this efficient mobility.[Key Words: Bacteriophage T4; nrdB intron; endonuclease mobility]

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