Use of a gene encoding a suppressor tRNA as a reporter of transcription: analyzing the action of the Nun protein of bacteriophage HK022

RNA polymerase elongation factors

The elongation phase of transcription by RNA polymerase is highly regulated and modulated. Both general and operon-specific elongation factors determine the local rate and extent of transcription to coordinate the appearance of transcript with its use as a messenger or functional ribonucleoprotein or regulatory element, as well as to provide operon-specific gene regulation.

Polynucleotide phosphorylase hinders mRNA degradation upon ribosomal protein S1 overexpression in Escherichia coli

The exoribonuclease polynucleotide phosphorylase (PNPase, encoded by pnp) is a major player in bacterial RNA decay. In Escherichia coli, PNPase expression is post-transcriptionally regulated at the level of mRNA stability. The primary transcript is very efficiently processed by the endonuclease RNase III at a specific site and the processed pnp mRNA is rapidly degraded in a PNPase-dependent manner. While investigating the PNPase autoregulation mechanism we found, by UV-cross-linking experiments, that the ribosomal protein S1 in crude extracts binds to the pnp-mRNA leader region. We assayed the potential role of S1 protein in pnp gene regulation by modulating S1 expression from depletion to overexpression. We found that S1 depletion led to a sharp decrease of the amount of pnp and other tested mRNAs, as detected by Northern blotting, whereas S1 overexpression caused a strong stabilization of pnp and the other transcripts. Surprisingly, mRNA stabilization depended on PNPase, as it was not observed in a pnp deletion strain. PNPase-dependent stabilization, however, was not detected by chemical decay assay of bulk mRNA. Overall, our data suggest that PNPase exonucleolytic activity may be modulated by the translation potential of the target mRNAs and that, upon ribosomal protein S1 overexpression, PNPase protects from degradation a set of full-length mRNAs. It thus appears that a single mRNA species may be differentially targeted to either decay or PNPase-dependent stabilization, thus preventing its depletion in conditions of fast turnover.

Protection of antiterminator RNA by the transcript elongation complex

Nascent transcripts encoded by the putL and putR sites of phage HK022 bind the transcript elongation complex and suppress termination at downstream transcription terminators. We report here that the chemical stability of putL RNA is considerably greater than that of the typical Escherichia coli message because the elongation complex protects this RNA from degradation. When binding to the elongation complex was prevented by mutation of either putL or RNA polymerase, RNA stability decreased more than 50-fold. The functional modification conferred by putL RNA on the elongation complex is also long-lived: the efficiency of terminator suppression remained high for at least 10 kb from the putL site. We find that RNase III rapidly and efficiently cleaved the transcript just downstream of the putL sequences, but such cleavage changed neither the stability of putL RNA nor the efficiency of antitermination. These results argue that the continuity of the RNA that connects put sequences to the growing point is not required for persistence of the antiterminating modification in vivo.

Conservation and diversity in the immunity regions of wild phages with the immunity specificity of phage lambda

The gene regulatory circuitry of phage lambda is among the best-understood circuits. Much of the circuitry centres around the immunity region, which includes genes for two repressors, CI and Cro, and their cis-acting sites. Related phages, termed lambdoid phages, have different immunity regions, but similar regulatory circuitry and genome organization to that of lambda, and show a mosaic organization, arising by recombination between lambdoid phages. We sequenced the immunity regions of several wild phages with the immunity specificity of lambda, both to determine whether natural variation exists in regulation, and to analyse conservation and variability in a region rich in well-studied regulatory elements. CI, Cro and their cis-acting sites are almost identical to those in lambda, implying that regulatory mechanisms controlled by the immunity region are conserved. A segment adjacent to one of the operator regions is also conserved, and may be a novel regulatory element. In most isolates, different alleles of two regulatory proteins (N and CII) flank the immunity region; possibly the lysis-lysogeny decision is more variable among isolates. Extensive mosaicism was observed for several elements flanking the immunity region. Very short sequence elements or microhomologies were also identified. Our findings suggest mechanisms by which fine-scale mosaicism arises.

Autogenous regulation of Escherichia coli polynucleotide phosphorylase during cold acclimation by transcription termination and antitermination

Adaptation of Escherichia coli at low temperature implicates a drastic reprogramming of gene expression patterns. Mechanisms operating downstream of transcription initiation, such as control of transcription termination, mRNA stability and translatability, play a major role in controlling gene expression in the cold acclimation phase. It was previously shown that Rho-dependent transcription termination within pnp, the gene encoding polynucleotide phosphorylase (PNPase), was suppressed in pnp nonsense mutants, whereas it was restored by complementation with wild type allele. Using a tRNA gene as a reporter and the strong Rho-dependent transcription terminator t ( imm ) of bacteriophage P4 as a tester, here we show that specific sites in the 5'-untranslated region of pnp mRNA are required for PNPase-sensitive cold-induced suppression of Rho-dependent transcription termination. We suggest that suppression of Rho-dependent transcription termination within pnp and its restoration by PNPase is an autogenous regulatory circuit that modulates pnp expression during cold acclimation.

Role of an RNase III binding site in transcription termination at lambda nutL by HK022 Nun protein

The phage HK022 Nun protein excludes phage lambda by binding nascent lambda pL and pR transcripts at nutL and nutR, respectively, and inducing transcription termination just downstream of these sites. Termination is more efficient at nutL than at nutR. One difference between nutL and nutR is the presence of RNase III processing sites (rIII) located immediately promoter distal to lambda nutL. We found that deletion of rIII dramatically reduced Nun transcription arrest in vitro but had little effect on termination in vivo. However, consistent with the in vitro results, overexpression of a transcript carrying nutL and rIII efficiently titrated Nun, allowing lambda to grow on a strain that expressed Nun, whereas a transcript carrying only nutL or nutL-rIII with nucleotides 97 to 141 deleted was ineffective. Rnc70, an RNase III mutant that binds but does not cleave rIII, also prevented Nun-mediated lambda exclusion. We propose that rIII enhances the on-rate of Nun at nutL, stimulating Nun-mediated arrest in vitro. We have shown that a specific element in rIII, i.e., box C (G89GUGUGUG), strongly enhances arrest on rIII+ templates. Nun-rIII interactions do not stimulate Nun termination in vivo, presumably because formation of the Nun-nutL complex is normally not rate-limiting in the cell. In contrast to Nun, N is not occluded by Rnc70 and is not efficiently titrated by a nutL-rIII transcript.

Dual role of boxB RNA motif in the mechanisms of termination/antitermination at the lambda tR1 terminator revealed in vivo

Rho-dependent transcription termination at the phage lambda tR1 terminator is governed primarily by the upstream rut element that encodes two RNA regions rutA and rutB. The two regions are separated by the boxB RNA motif, which is believed to be dispensable for Rho activity but serves as a binding site for lambda N protein in the antitermination process. By using a minimal in vivo termination system, we show that the intervening boxB RNA motif has a double function in the mechanisms of termination/antitermination at lambdatR1. As a folded hairpin structure, it acts as a clamp that holds rutA and rutB side by side for optimal interactions with Rho leading to efficient termination. Conversely, the binding of N protein to boxB induces antitermination at lambdatR1 by preventing access of Rho to the rut sequences. This dual role was clearly shown in vivo by studying the effects of multiple mutations within the boxB hairpin stem on transcription termination and by substituting the N/boxB couple with the unrelated coat protein of phage MS2 and its stem-loop RNA binding site.

Suppression of factor-dependent transcription termination by antiterminator RNA

Nascent transcripts of the phage HK022 put sites modify the transcription elongation complex so that it terminates less efficiently at intrinsic transcription terminators and accelerates through pause sites. We show here that the modification also suppresses termination in vivo at two factor-dependent terminators, one that depends on the bacterial Rho protein and a second that depends on the HK022-encoded Nun protein. Suppression was efficient when the termination factors were present at physiological levels, but an increase in the intracellular concentration of Nun increased termination both in the presence and absence of put. put-mediated antitermination thus shows no apparent terminator specificity, suggesting that put inhibits a step that is common to termination at the different types of terminator.

Role of E.coli transcription-repair coupling factor Mfd in Nun-mediated transcription termination

Phage HK022 Nun protein excludes phage lambda by binding nascent lambda-nut RNA and inducing termination and transcript release. In contrast, in a purified in vitro system, Nun arrests transcription on lambdaDNA templates without dissociation of the transcription elongation complex (TEC). Our evidence indicates that transcription-repair coupling factor (Mfd) frees Nun-arrested RNA polymerase. The activity of Nun is enhanced in an mfd-null mutant, consistent with prolonged association of Nun with the TEC. Furthermore, expression of lambda nut RNA in the mfd mutant titrates Nun, allowing superinfecting lambda to form plaques. Finally, addition of Mfd releases a Nun-arrested transcription complex in vitro.

Phage HK022 Nun protein represses translation of phage lambda N (transcription termination/translation repression)

The N-terminal arginine-rich motif of phage HK022 Nun protein binds to NUT sequences in phage lambda nascent transcripts and induces transcription termination. Interactions between the Nun C terminus and RNA polymerase as well as the DNA template are required for termination. We have isolated Nun C-terminal point and deletion mutants that are unable to block transcription. The mutants bind NUT RNA and inhibit translation of the lambda N gene. Thus HK022 excludes lambda both by terminating transcription on the phage chromosome and by preventing translation of the essential lambda N gene. Like N autoregulation, translation repression by Nun requires host RNaseIII deficiency (rnc) or a mutation in the RNaseIII processing site (rIII) located between NUTL and the beginning of the N coding sequence. Our data support the idea that Nun bound at NUTL causes steric interference with ribosome attachment to the nearby N coding sequence. Two models, Nun acting alone or in complex with host proteins, are discussed.

Transcription termination and anti-termination in E. coli

Transcription termination in Escherichia coli is controlled by many factors. The sequence of the DNA template, the structure of the transcript, and the actions of auxiliary proteins all play a role in determining the efficiency of the process. Termination is regulated and can be enhanced or suppressed by host and phage proteins. This complex reaction is rapidly yielding to biochemical and structural analysis of the interacting factors. Below we review and attempt to unify into basic principles the remarkable recent progress in understanding transcription termination and anti-termination.

Transcription attenuation associated with bacterial repetitive extragenic BIME elements

Transcription attenuation comprises several processes that affect transcript elongation and transcription termination, and has an important role in regulating gene expression. In most cases, transcription attenuation is used as a regulatory mechanism that allows the cell to adjust protein synthesis levels in response to a specific signal. Here, by using a tRNA gene as a transcriptional reporter, we characterize a new type of transcription attenuation mechanism in Escherichia coli that involves bacterial interspersed mosaic elements (BIMEs), the main family of repetitive extragenic elements. The transcription termination factor Rho is required for attenuation in association with BIMEs, thus revealing a new role for Rho as a BIMEs-dependent global regulator of gene expression. By mutational analyses, we identified nucleotide determinants of BIMEs that are required for attenuation and showed that this process relies on a sequence-specific mechanism. Our data are consistent with a model in which BIMEs provoke a pause in RNA polymerase movement and Rho acts ultimately to terminate transcription. BIME-dependent transcription attenuation may be used as a means to differentially regulate expression of adjacent genes belonging to a single operon. BIMEs are dispersed in more than 250 operons such that attenuation can simultaneously affect expression of a large number of genes encoding unrelated proteins. This attenuation phenomenon, together with the known ability of BIMEs to stabilize upstream mRNA, reveals how dispersion of these abundant repetitive elements may affect gene regulation at the genome level.

Antisense RNA-dependent transcription termination sites that modulate lysogenic development of satellite phage P4

In the lysogenic state, bacteriophage P4 prevents the expression of its own replication genes, which are encoded in the left operon, through premature transcription termination. The phage factor responsible for efficient termination is a small, untranslated RNA (CI RNA), which acts as an antisense RNA and controls transcription termination by pairing with two complementary sequences (seqA and seqC) located within the leader region of the left operon. A Rho-dependent termination site, timm, was previously shown to be involved in the control of P4 replication gene expression. In the present study, by making use of phage PhiR73 as a cloning vector and of suppressor tRNAGly as a reporter gene, we characterized two additional terminators, t1 and t4. Although transcription termination at neither site requires the Rho factor, only t1 has the typical structure of a Rho-independent terminator. t1 is located between the PLE promoter and the cI gene, whereas t4 is located between cI and timm. Efficient termination at t1 requires the CI RNA and the seqA target sequence; in vitro, the CI RNA enhanced termination at t1 in the absence of any bacterial factor. A P4 mutant, in which the t1 terminator has been deleted, can still lysogenize both Rho+ and Rho- strains and exhibits increased expression of CI RNA. These data indicate that t1 and the Rho-dependent timm terminators are not essential for lysogeny. t1 is involved in CI RNA autoregulation, whereas t4 appears to be the main terminator necessary to prevent expression of the lytic genes in the lysogenic state.

Transcriptional and post-transcriptional control of polynucleotide phosphorylase during cold acclimation in Escherichia coli

Polynucleotide phosphorylase (PNPase, polyribonucleotide nucleotidyltransferase, EC 2.7.7.8) is one of the cold shock-induced proteins in Escherichia coli and pnp, the gene encoding it, is essential for growth at low temperatures. We have analysed the expression of pnp upon cold shock and found a dramatic transient variation of pnp transcription profile: within the first hour after temperature downshift the amount of pnp transcripts detectable by Northern blotting increased more than 10-fold and new mRNA species that cover pnp and the downstream region, including the cold shock gene deaD, appeared; 2 h after temperature downshift the transcription profile reverted to a preshift-like pattern in a PNPase-independent manner. The higher amount of pnp transcripts appeared to be mainly due to an increased stability of the RNAs. The abundance of pnp transcripts was not paralleled by comparable variation of the protein: PNPase steadily increased about twofold during the first 3 h at low temperature, as determined both by Western blotting and enzymatic activity assay, suggesting that PNPase, unlike other known cold shock proteins, is not efficiently translated in the acclimation phase. In experiments aimed at assessing the role of PNPase in autogenous control during cold shock, we detected a Rho-dependent termination site within pnp. In the cold acclimation phase, termination at this site depended upon the presence of PNPase, suggesting that during cold shock pnp is autogenously regulated at the level of transcription elongation.

Family values in the age of genomics: comparative analyses of temperate bacteriophage HK022

HK022 is a temperate coliphage related to phage lambda. Its chromosome has been completely sequenced, and several aspects of its life cycle have been intensively studied. In the overall arrangement, expression, and function of most of its genes, HK022 broadly resembles lambda and other members of the lambda family. Upon closer view, significant differences emerge. The differences reveal alternative strategies used by related phages to cope with similar problems and illuminate previously unknown regulatory and structural motifs. HK022 prophages protect lysogens from superinfection by producing a sequence-specific RNA binding protein that prematurely terminates nascent transcripts of infecting phage. It uses a novel RNA-based mechanism to antiterminate its own early transcription. The HK022 protein shell is strengthened by a complex pattern of covalent subunit interlinking to form a unitary structure that resembles chain-mail armour. Its integrase and repressor proteins are similar to those of lambda, but the differences provide insights into the evolution of biological specificity and the elements needed for construction of a stable genetic switch.

Constitutive expression of a transcription termination factor by a repressed prophage: promoters for transcribing the phage HK022 nun gene

Lysogens of phage HK022 are resistant to infection by phage lambda. Lambda resistance is caused by the action of the HK022 Nun protein, which prematurely terminates early lambda transcripts. We report here that transcription of the nun gene initiates at a constitutive prophage promoter, P(Nun), located just upstream of the protein coding sequence. The 5' end of the transcript was determined by primer extension analysis of RNA isolated from HK022 lysogens or RNA made in vitro by transcribing a template containing the promoter with purified Escherichia coli RNA polymerase. Inactivation of P(Nun) by mutation greatly reduced Nun activity and Nun antigen in an HK022 lysogen. However, a low level of residual activity was detected, suggesting that a secondary promoter also contributes to nun expression. We found one possible secondary promoter, P(Nun)', just upstream of P(Nun). Neither promoter is likely to increase the expression of other phage genes in a lysogen because their transcripts should be terminated downstream of nun. We estimate that HK022 lysogens in stationary phase contain several hundred molecules of Nun per cell and that cells in exponential phase probably contain fewer.

Translation of two nested genes in bacteriophage P4 controls immunity-specific transcription termination

In phage P4, transcription of the left operon may occur from both the constitutive PLE promoter and the regulated PLL promoter, about 400 nucleotides upstream of PLE. A strong Rho-dependent termination site, timm, is located downstream of both promoters. When P4 immunity is expressed, transcription starting at PLE is efficiently terminated at timm, whereas transcription from PLL is immunity insensitive and reads through timm. We report the identification of two nested genes, kil and eta, located in the P4 left operon. The P4 kil gene, which encodes a 65-amino-acid polypeptide, is the first translated gene downstream of the PLE promoter, and its expression is controlled by P4 immunity. Overexpression of kil causes cell killing. This gene is the terminal part of a longer open reading frame, eta, which begins upstream of PLE. The eta gene is expressed when transcription starts from the PLL promoter. Three likely start codons predict a size between 197 and 199 amino acids for the Eta gene product. Both kil and eta overlap the timm site. By cloning kil upstream of a tRNA reporter gene, we demonstrated that translation of the kil region prevents premature transcription termination at timm. This suggests that P4 immunity might negatively control kil translation, thus enabling transcription termination at timm. Transcription starting from PL proceeds through timm. Mutations that create nonsense codons in eta caused premature termination of transcription starting from PLL. Suppression of the nonsense mutation restored transcription readthrough at timm. Thus, termination of transcription from PLL is prevented by translation of eta.

Escherichia coli nusG mutations that block transcription termination by coliphage HK022 Nun protein

The Escherichia coli nusG gene product is required for transcription termination by phage HK022 Nun protein at the lambda nutR site in vivo. We show that it is also essential for Nun termination at lambda nutL. Three recessive mis-sense nusG mutations have been isolated that inhibit termination by Nun at lambda nutR. The mutations are ineffective in a lambda pL nutL fusion, even when lambda nutR replaces lambda nutL. The mutant strains support lambda growth, indicating that lambda N antitermination activity is not impaired. Transcription arrest by Nun in vitro is stimulated by NusG protein at both lambda nutR and lambda nutL. Mutant NusG protein fails to enhance transcriptional arrest by Nun at either site. The mutant protein, like the wild-type protein, suppresses transcriptional pausing by RNA polymerase and stimulates Rho-dependent termination. These results imply that the role of NusG in Nun termination may be distinct from its roles in other transcription reactions.

The Nun protein of bacteriophage HK022 inhibits translocation of Escherichia coli RNA polymerase without abolishing its catalytic activities

Bacteriophage HK022 Nun protein blocks transcription elongation by Escherichia coli RNA polymerase in vitro without dissociating the transcription complex. Nun is active on complexes located at any template site tested. Ultimately, only the 3'-OH terminal nucleotide of the nascent transcript in an arrested complex can turn over; it is removed by pyrophosphate and restored with NTPs. This suggests that Nun inhibits the translocation of RNA polymerase without abolishing its catalytic activities. Unlike spontaneously arrested complexes, Nun-arrested complexes cannot be reactivated by transcription factor GreB. The various complexes show distinct patterns of nucleotide incorporation and pyrophosphorolysis before or after treatment with Nun, suggesting that the configuration of RNAP, transcript, and template DNA is different in each complex.

Interaction between the phage HK022 Nun protein and the nut RNA of phage lambda

The nun gene product of prophage HK022 excludes phage lambda infection by blocking the expression of genes downstream from the lambda nut sequence. The Nun protein functions both by competing with lambda N transcription-antitermination protein and by actively inducing transcription termination on the lambda chromosome. We demonstrate that Nun binds directly to a stem-loop structure within nut RNA, boxB, which is also the target for the N antiterminator. The two proteins show comparable affinities for boxB and they compete with each other. Their interactions with boxB are similar, as shown by RNase protection experiments, NMR spectroscopy, and analysis of boxB mutants. Each protein binds the 5' strand of the boxB stem and the adjacent loop. The stem does not melt upon the binding of Nun or N, as the 3' strand remains sensitive to a double-strand-specific RNase. The binding of RNA partially protects Nun from proteolysis and changes its NMR spectra. Evidently, although Nun and N bind to the same surface of boxB RNA, their respective complexes interact differently with RNA polymerase, inducing transcription termination or antitermination, respectively.