Transcription of a bacteriophage lambda DNA site blocks growth of Escherichia coli

Evidence of bar minigene expression and tRNA2Ile sequestration as peptidyl-tRNA2Ile during lambda bacteriophage development

Lambda bacteriophage development is impaired in Escherichia coli cells defective for peptidyl (pep)-tRNA hydrolase (Pth). Single-base-pair mutations (bar(-)) that affect translatable two-codon open reading frames named bar minigenes (barI or barII) in the lambda phage genome promote the development of this phage in Pth-defective cells (rap cells). When the barI minigene is cloned and overexpressed from a plasmid, it inhibits protein synthesis and cell growth in rap cells by sequestering tRNA(2)(Ile) as pep-tRNA(2)(Ile). Either tRNA(2)(Ile) or Pth may reverse these effects. In this paper we present evidence that both barI and barII minigenes are translatable elements that sequester tRNA(2)(Ile) as pep-tRNA(2)(Ile). In addition, overexpression of the barI minigene impairs the development even of bar(-) phages in rap cells. Interestingly, tRNA or Pth may reestablish lambda phage development. These results suggest that lambda bar minigenes are expressed and tRNA(2)(Ile) is sequestered as pep-tRNA(2)(Ile) during lambda phage development.

Increased bar minigene mRNA stability during cell growth inhibition

Bacteriophage lambda is unable to grow vegetatively on Escherichia coli mutants defective in peptidyl-tRNA hydrolase (Pth) activity. Mutations which allow phage growth on the defective host have been located at regions named bar in the lambda genome. Expression of wild-type bar regions from plasmid constructs results in inhibition of protein synthesis and lethality to Pth-defective cells. Two of these wild-type bar regions, barI+ and barII+, contain minigenes with similar AUG-AUA-stop codon sequences preceded by different Shine-Dalgarno (SD) and spacer regions. The induced expression of barI+ and barII+ regions from plasmid constructs resulted in similar patterns of protein synthesis inhibition and cell growth arrest. Therefore, these deleterious effects may stem from translation of the transcripts containing the minigene two-codon 'ORF' (open reading frame). To test for this possibility, we assayed the effect of point mutations within the barI minigene. The results showed that a base pair substitution within the SD and the two-codon 'ORF' sequences affected protein synthesis and cell growth inhibition. In addition, mRNA stability was altered in each mutant. Higher mRNA stability correlated with the more toxic minigenes. We argue that this effect may be caused by ribosome protection of the mRNA in paused complexes as a result of deficiency of specific tRNA.

Shutdown in protein synthesis due to the expression of mini-genes in bacteria

Mutants of Escherichia coli partially deficient in peptidyl-tRNA hydrolase are killed by the expression of certain very short open reading frames (mini-genes), encoded by the wild-type bar regions of phage lambda. According to the current hypothesis, protein synthesis is shut off, and the host cells die, after essential tRNA species become sequestered due to abnormal translation termination (drop-off) of mini-gene-encoded peptides as peptidyl-tRNA. Here we study variants of bar mini-genes, both in vivo and in vitro, in order to identify the structural elements that influence this inhibition of protein synthesis. Three parameters were measured during the expression of these variants: the rates of normal translation termination, peptidyl-tRNA dissociation from the ribosome and hydrolysis of peptidyl-tRNA by peptidyl-tRNA hydrolase were measured. Previous observations that RRF, EF-G and RF3 stimulated drop-off were confirmed and extended; stimulation by these factors can reach 30-fold. Both factor-stimulated and spontaneous drop-off depended on the nature of the stop signal. The degree of inhibition of cell growth following induction of mini-gene expression could be accounted for in terms of a toxicity index comprising the three parameters above. Inhibition was greatly reduced in cells lacking RF3. Mini-genes with more efficient Shine/Dalgarno sequences killed cells even with normal peptidyl-tRNA hydrolase activity. It is proposed that the retranslation by ribosomes of mini-gene transcripts with efficient ribosome binding (Shine/Dalgarno) sequences strongly contributes to the inhibitory effects of mini-gene expression on protein synthesis.

Inhibition of translation and cell growth by minigene expression

A random five-codon gene library was used to isolate minigenes whose expression causes cell growth arrest. Eight different deleterious minigenes were isolated, five of which had in-frame stop codons; the predicted expressed peptides ranged in size from two to five amino acids. Mutational analysis demonstrated that translation of the inhibitory minigenes is essential for growth arrest. Pulse-labeling experiments showed that expression of at least some of the selected minigenes results in inhibition of cellular protein synthesis. Expression of the deleterious minigenes in cells deficient in peptidyl-tRNA hydrolase causes accumulation of families of peptidyl-tRNAs corresponding to the last minigene codon; the inhibitory action of minigene expression could be suppressed by overexpression of the tRNA corresponding to the last sense codon in the minigene. Experimental data are compatible with the model that the deleterious effect of minigene expression is mediated by depletion of corresponding pools of free tRNAs.

lambda bar minigene-mediated inhibition of protein synthesis involves accumulation of peptidyl-tRNA and starvation for tRNA

Expression of the bacteriophage lambda two-codon, AUG AUA, barI minigene (bar+) leads to the arrest of protein synthesis in cells defective in peptidyl-tRNA hydrolase (Pth). It has been hypothesized that translation of the bar+ transcript provokes premature release and accumulation of peptidyl-tRNA (p-tRNA). Inhibition of protein synthesis would then result from either starvation of sequestered tRNA or from toxicity of accumulated p-tRNA. To test this hypothesis and to investigate the cause of arrest, we used a coupled in vitro transcription-translation system primed with DNA containing bar+ and the beta-lactamase-encoding gene of the vector as a reporter. The results show that expression of bar+ minigene severely inhibits beta-lactamase polypeptide synthesis by Pth-defective extracts and partially inhibits synthesis by wild-type extracts. Fractions enriched for Pth, or a homogeneous preparation of Pth, prevented and reversed bar+-mediated inhibition. A mutant minigene, barA702, which changes the second codon AUA (Ile) to AAA (Lys), was also toxic for Pth-defective cells. Expression of barA702 inhibited in vitro polypeptide synthesis by Pth-defective extracts and, as with bar+, exogenous Pth prevented inhibition. Addition of pure tRNALys prevented inhibition by barA702 but not by bar+. Expression of bar+ and barA702 led to release and accumulation of p-tRNAIle and p-tRNALys respectively but bar+ also induced accumulation of p-tRNALys. Finally, bar+ stimulated association of methionine with ribosomes probably as fMet-tRNAfMet and the accumulation of methionine and isoleucine in solution as peptidyl-tRNA (p-tRNA). These results indicate that minigene-mediated inhibition of protein synthesis involves premature release of p-tRNA, misincorporation of amino acyl-tRNA, accumulation of p-tRNAs and possibly sequestration of tRNAs.

Regulation of protein synthesis by minigene expression

Peptidyl-tRNA hydrolase (Pth), an enzyme essential for Escherichia coli viability, scavenges peptidyl-tRNA released during abortive polypeptide chain elongation. Bacterial strains of E coli partially defective in Pth activity are unable to maintain bacteriophage lambda growth. Phage mutations that overcome the bacterial defect have been located to several regions in the lambda genome named bar. Plasmid constructs expressing just the bar region are toxic and cause a general arrest of protein synthesis in Pth-defective cells. Inspection of the nucleotide sequence from two bar regions reveals the short coding sequence AUG AUA Stop, spaced by an AT-rich segment from a Shine Dalgarno-like sequence (S-D). These sequences have been named minigenes. Base changes altering the putative S-D, the two sense codons, or the stop codon have been found to reduce Bar-toxicity. Transcripts containing bar function as mRNA. Upon expression in pth mutants, wild-type (bar+) transcripts are found associated with ribosomes. In addition, bar+ RNA forms ternary complexes with the 30S ribosomal subunit and the initiator tRNA and can be released upon run-off translation in the same way as an authentic mRNA. A cell free system for protein synthesis reproduces the in vivo effects: bar+ expression inhibits protein synthesis, bar+ RNA sequences are associated with ribosomes in the inhibited extracts, addition of purified Pth restores synthesis, and excess of tRNA(Lys), specific for the last sense codon in a mutant toxic minigene, prevents protein synthesis inhibition. Also, bar expression promotes association of methionine with ribosomes possibly in a translation complex. These results are consistent with a model proposing tRNA starvation to explain the behaviour of a pth mutant, thermosensitive for protein synthesis.

Inhibition of Escherichia coli protein synthesis by abortive translation of phage lambda minigenes

Escherichia coli mutants defective in peptidyl-tRNA hydrolase activity are unable to maintain bacteriophage lambda vegetative growth. Phage mutants, named bar, overcome the host limitation to support viral growth. Multicopy expression of lambda wild-type bar regions is deleterious to hydrolase-defective cells because it provokes arrest of protein synthesis. We noticed that the bar regions include minigenes whose transcripts would contain a Shine-Dalgarno-like sequence appropriately spaced for translation from a two codon open reading frame. To investigate the mechanism of bar inhibition, we asked if transcripts of the barI region function as mRNAs in their ribosomal interactions. We found that bar-containing RNA associates with ribosomes, forms ternary initiation complexes, yields a toeprint signal, and can be removed from ribosomes by run-off translation, as authentic mRNA. Since bar-containing RNA has the properties of a messenger, we propose that its translation leads to drop-off and accumulation of peptidyl-tRNA in pth-defective cells. Starvation of the tRNA(s) sequestered in pepidyl-tRNA(s) eventually causes inhibition of protein synthesis.

Overexpression of an mRNA dependent on rare codons inhibits protein synthesis and cell growth

lambda's int gene contains an unusually high frequency of the rare arginine codons AGA and AGG, as well as dual rare Arg codons at three positions. Related work has demonstrated that Int protein expression depends on the rare AGA tRNA. Strong transcription of the int mRNA with a highly efficient ribosome-binding site leads to inhibition of Int protein synthesis, alteration of the overall pattern of cellular protein synthesis, and cell death. Synthesis or stability of int and ampicillin resistance mRNAs is not affected, although a portion of the untranslated int mRNA appears to be modified in a site-specific fashion. These phenotypes are not due to a toxic effect of the int gene product and can be largely reversed by supplementation of the AGA tRNA in cells which bear plasmids expressing the T4 AGA tRNA gene. This indicates that depletion of the rare Arg tRNA due to ribosome stalling at multiple AGA and AGG codons on the overexpressed int mRNA underlies all of these phenomena. It is hypothesized that int mRNA's effects on protein synthesis and cell viability relate to phenomena involved in lambda phage induction and excision.

Open reading frames flanking the peptidyl-tRNA hydrolase-encoding gene of Escherichia coli

The nucleotide (nt) sequences flanking the peptidyl-tRNA hydrolase-encoding gene (pth) of Escherichia coli were determined and analyzed. A coding open reading frame (ORF-3), identified just downstream from pth, had a deduced amino acid (aa) sequence homologous to a family of GTP-binding proteins. We found discrepancies between the ORF-3 sequence from a plasmid clone used in previous studies and another one derived from Kohara's phage collection. Two putative genes, ORF-4 and ORF-2, were also found upstream from pth. ORF-4 could code for a 393-aa peptide homologous to membrane-bound proteins. The nt sequence between ORF-2 and pth revealed the existence of a CAP-binding site correctly positioned to regulate the expression of ORF-2.

Ribosomal RNA and peptidyl-tRNA hydrolase: a peptide chain termination model for lambda bar RNA inhibition

We propose here a model to explain the inhibition of bacteriophage lambda (lambda) vegetative growth and the killing of E coli cells defective in peptidyl-tRNA hydrolase (Pth) by lambda bar RNA. The model suggests that bar RNA, which contains a characteristic UGA triplet, base-pairs in an anti-parallel fashion with the 1199-1205 region of E coli 16S rRNA. In doing so, it prevents the required functioning of that region of 16S rRNA in UGA-specific peptide chain termination. Pth is implicated in peptide chain termination because a defect in Pth is required for the achievement of the bar RNA inhibitory effects. We make certain predictions that flow from the model, predictions involving suppression of nonsense mutations, and present preliminary experimental results that demonstrate the fulfillment of those predictions.

A short DNA sequence from lambda phage inhibits protein synthesis in Escherichia coli rap

The Escherichia coli rap mutant inhibits vegetative growth of bacteriophage lambda. Phage mutations termed bar, which overcome the rap defect, have been mapped to three genetic loci in the pL operon. Plasmids with a lambda wild-type bar DNA segment cloned downstream from an active promoter cannot be maintained in rap mutant bacteria. The viability of a rap mutant strain decreases rapidly after induction of transcription through bar regions present on plasmids. Under these (restrictive) conditions the expression of plasmid-encoded beta-lactamase and plasmid DNA replication are arrested, but plasmid RNA synthesis continues for several hours. Analysis of protein extracts from E. coli rap cells containing bar plasmids revealed that both plasmid and bacterial protein synthesis are inhibited under restrictive conditions. In addition, unlike other RNAs tested, the chemical half-life of bar RNA increases 3.5-fold relative to the half-life of bar RNA under permissive conditions. We propose that transcription through the bar region, or the accumulation of bar RNA, results in an irreversible defect in cellular mRNA translation. This defect eventually kills the rap cells, and thus prevents bar plasmid maintenance.