In Escherichia coli, programmed cell death is mediated through ''addiction modules'' consisting of two genes; the product of one gene is long-lived and toxic, whereas the product of the other is short-lived and antagonizes the toxic effect. Here we show that the product of rexB, one of the few genes expressed in the lysogenic state of bacteriophage , prevents cell death directed by each of two addiction modules, phd-doc of plasmid prophage P1 and the rel mazEF of E. coli, which is induced by the signal molecule guanosine 3,5-bispyrophosphate (ppGpp) and thus by amino acid starvation. RexB inhibits the degradation of the antitoxic labile components Phd and MazE of these systems, which are substrates of ClpP proteases. We present a model for this anti-cell death effect of RexB through its action on the ClpP proteolytic subunit. We also propose that the rex operon has an additional function to the well known phenomenon of exclusion of other phages; it can prevent the death of lysogenized cells under conditions of nutrient starvation. Thus, the rex operon may be considered as the ''survival operon'' of phage .

The rex operon of bacteriophage A excludes the development of several unrelated bacteriophages. Here we present an additional A rexB function: it prevents degradation of the short-lived protein A 0 known to be involved in A DNA replication. We have shown that it is the productof rexB that is responsible for the stabilization of A 0: when a nonsense mutation is present in rexB, A 0 protein is labile; suppression of the mutation by the corresponding nonsense suppressor causes partial restabilization of A 0. A rexB also stabilizes A 0 in trans. We discuss our results in relation to the function of rexB in-A DNA replication and'its role in the protein degradation pathways of bacteriophage A.When phage A is in the lysogenic state in its host Escherichia coli, the only phage genes expressed are the adjacent genes cI and rex (1). The product of cI, the A repressor, prevents vegetative development of the prophage and also further infection by homologous phages (2). A rex expresses the Rex function, shown to exclude the development of several unrelated phages (3-11).The first-described Rex function was the exclusion by A prophage of the development of phage T4 rII mutants. Rex does not exclude wild-type T4 (3). The system for T4 ril exclusion (rex) is a landmark in the history of molecular biology: it was used for the first fine-structure analysis of a genetic region (T4rII) (3), for defining the cistron (12), and also for elucidation of the triplet nature of the genetic code (13). Later, the A rex exclusion function was found to include the restriction of mutants of other phages as well as of T4 (6-11). Overexpression of the rex function causes exclusion of the development of wild-type phages (14). Furthermore, A rex overexpression will inhibit the function of the E. coli host even without superinfection (15).The rex exclusion function is performed by the products of two adjacent genes, rexA and rexB (16,17). The genes rexA and rexB can be expressed coordinately with the A cI repressor gene from promoters PRM and PRE (16,18). There is a third promoter, PLIT, which overlaps the region encoding the carboxyl terminus of rexA (Fig. 1A). Transcription from PLIT results in a 470-nucleotide-long lit mRNA that permits expression of rexB without that of rexA (16,20). When A DNA replication is initiated at the A origin, >10-fold increase in lit mRNA transcription has been detected (20). This shift from coordinate to discoordinate expression of rexB over rexA implies that A rexB has another function, perhaps connected to A DNA replication, and independent of that of rexA (16).Hfere we report an additional function for the product of A rexB: it prevents degradation of the A 0 protein. A 0 is a short-lived protein involved in A DNA replication (21)(22)(23)(24)(25). We shall discuss our results both in relation to the role ofrexB in A DNA replication and in relation to the mechanism of protein degradation-antidegradation as a regulatory device in A development. CSR6O3 (recAl, uvrA6, phr-J, supE44, thr-1, leuB6, proA2, argE3...

The genetic code, once thought to be rigid, has been found to be quite flexible, permitting several different reading alternatives. One of these is translational frameshifting, a process programmed in the mRNA sequence and which enables a +1 or -1 shift from the reading frame of the initiation codon. So far, the involvement of translational frameshifting in gene expression has been described mainly in viruses (particularly retroviruses), retrotransposons, and bacterial insertion elements. In this MicroReview, we present a survey of the cellular genes, mostly in Escherichia coli, which have been found to be expressed through a translational frameshifting process, as well as a discussion of the regulatory implications of this process.

Protein 0 of bacteriophage A is a short-lived protein which has a key role in the replication of the phage DNA in Escherichia coli. Here we present evidence that XO degradation is energy dependent: it is impaired by cyanide and a-methylglucoside, both of which inhibit cellular energy metabolism. Removal of these inhibitors restored the degradation of XO. Our experiments suggest that limited Amounts of cellular energy are sufficient to support XO degradation. In addition, degradation of XO protein is prevented by a mutation in the E. coli clpP gene, but not by a mutation in the clpA gene. These results suggest that the ClpP protease is involved in the energy-dependent degradation of the XO protein.Most proteins in Escherichia coli are relatively stable, with half-lives that are greater than the doubling time of the cell (9). In contrast, a number of proteins in the E. coli cell have been found to be highly unstable. These proteins are subjected to rapid degradation by various proteases, some of which are energy dependent (12). Among the proteins which are subjected to rapid degradation in E. coli are several proteins of bacteriophage X which regulate the life cycle of the phage (6,13,14,19,25,26). For example, the antitermination protein N of bacteriophage X is degraded by the product of the E. coli lon gene, which is an ATP-dependent protease (22). On the other hand, XcII protein, which has a key role in the decision between the phage's lytic or lysogenic cycles, is subjected to degradation by the Hfl system, which is energy independent (2, 12). Several other X proteins are also subjected to proteolysis through as yet unidentified degradation pathways. These include the short-lived XO protein, which has a key role in the phage X DNA replication (1,5,7,27,28). Degradation of XO protein is prevented by the E. coli rexB gene product (24). Here, we present evidence that the rapid degradation of XO in E. coli is energy dependent. In addition, our results support the findings that the degradation of XO is not prevented in an E. coli strain carrying a mutation in the lon gene (11). However, XO degradation is inhibited in E. coli cells carrying a mutation in the clpP gene. Thus, our results suggest that the product of the clpP gene, which has a proteolytic activity, is involved in the energy-dependent degradation of the XO protein.Effect of energy metabolism inhibitors on XO degradation. XO protein is rapidly degraded in E. coli cells (20,21). We have previously studied XO degradation by using plasmid pRS4, which carries the XO gene under the control of the APL promoter (24). Here, using the same experimental system, we examined whether metabolic energy is required for the degradation of XO. For this purpose, we studied the fate of the XO protein in E. coli cells under conditions of impaired cellular energy production. XO protein was labelled for 2 min in E. coli CSR603(XcI857Sam7) carrying plasmid pRS4 (Table 1) grown in the presence of glucose, as described previously (24). The cells were washed and suspended in a glu...

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