Nus Factors of Escherichia coli

AtNusG, a chloroplast nucleoid protein of bacterial origin linking chloroplast transcriptional and translational machineries, is required for proper chloroplast gene expression in Arabidopsis thaliana

In Escherichia coli, transcription-translation coupling is mediated by NusG. Although chloroplasts are descendants of endosymbiotic prokaryotes, the mechanism underlying this coupling in chloroplasts remains unclear. Here, we report transcription-translation coupling through AtNusG in chloroplasts. AtNusG is localized in chloroplast nucleoids and is closely associated with the chloroplast PEP complex by interacting with its essential component PAP9. It also comigrates with chloroplast ribosomes and interacts with their two components PRPS5 (uS5c) and PRPS10 (uS10c). These data suggest that the transcription and translation machineries are coupled in chloroplasts. In the atnusg mutant, the accumulation of chloroplast-encoded photosynthetic gene transcripts, such as psbA, psbB, psbC and psbD, was not obviously changed, but that of their proteins was clearly decreased. Chloroplast polysomic analysis indicated that the decrease in these proteins was due to the reduced efficiency of their translation in this mutant, leading to reduced photosynthetic efficiency and enhanced sensitivity to cold stress. These data indicate that AtNusG-mediated coupling between transcription and translation in chloroplasts ensures the rapid establishment of photosynthetic capacity for plant growth and the response to environmental changes. Therefore, our study reveals a conserved mechanism of transcription-translation coupling between chloroplasts and E. coli, which perhaps represents a regulatory mechanism of chloroplast gene expression. This study provides insights into the underlying mechanisms of chloroplast gene expression in higher plants.

Coupled Transcription-Translation in Prokaryotes: An Old Couple With New Surprises

Coupled transcription-translation (CTT) is a hallmark of prokaryotic gene expression. CTT occurs when ribosomes associate with and initiate translation of mRNAs whose transcription has not yet concluded, therefore forming "RNAP.mRNA.ribosome" complexes. CTT is a well-documented phenomenon that is involved in important gene regulation processes, such as attenuation and operon polarity. Despite the progress in our understanding of the cellular signals that coordinate CTT, certain aspects of its molecular architecture remain controversial. Additionally, new information on the spatial segregation between the transcriptional and the translational machineries in certain species, and on the capability of certain mRNAs to localize translation-independently, questions the unanimous occurrence of CTT. Furthermore, studies where transcription and translation were artificially uncoupled showed that transcription elongation can proceed in a translation-independent manner. Here, we review studies supporting the occurrence of CTT and findings questioning its extent, as well as discuss mechanisms that may explain both coupling and uncoupling, e.g., chromosome relocation and the involvement of cis- or trans-acting elements, such as small RNAs and RNA-binding proteins. These mechanisms impact RNA localization, stability, and translation. Understanding the two options by which genes can be expressed and their consequences should shed light on a new layer of control of bacterial transcripts fate.

Autoregulation of greA Expression Relies on GraL Rather than on greA Promoter Region

GreA is a well-characterized transcriptional factor that acts primarily by rescuing stalled RNA polymerase complexes, but has also been shown to be the major transcriptional fidelity and proofreading factor, while it inhibits DNA break repair. Regulation of greA gene expression itself is still not well understood. So far, it has been shown that its expression is driven by two overlapping promoters and that greA leader encodes a small RNA (GraL) that is acting in trans on nudE mRNA. It has been also shown that GreA autoinhibits its own expression in vivo. Here, we decided to investigate the inner workings of this autoregulatory loop. Transcriptional fusions with lacZ reporter carrying different modifications (made both to the greA promoter and leader regions) were made to pinpoint the sequences responsible for this autoregulation, while GraL levels were also monitored. Our data indicate that GreA mediated regulation of its own gene expression is dependent on GraL acting in cis (a rare example of dual-action sRNA), rather than on the promoter region. However, a yet unidentified, additional factor seems to participate in this regulation as well. Overall, the GreA/GraL regulatory loop seems to have unique but hard to classify properties.

Regulated chloroplast transcription termination

Transcription termination by the RNA polymerase (RNAP) is a fundamental step of gene expression that involves the release of the nascent transcript and dissociation of the RNAP from the DNA template. However, the functional importance of termination extends beyond the mere definition of the gene borders. Chloroplasts originate from cyanobacteria and possess their own gene expression system. Plastids have a unique hybrid transcription system consisting of two different types of RNAPs of dissimilar phylogenetic origin together with several additional nuclear encoded components. Although the basic components involved in chloroplast transcription have been identified, little attention has been paid to the chloroplast transcription termination. Recent identification and functional characterization of novel factors in regulating transcription termination in Arabidopsis chloroplasts via genetic and biochemical approaches have provided insights into the mechanisms and significance of transcription termination in chloroplast gene expression. This review provides an overview of the current knowledge of the transcription termination in chloroplasts.

Rho Protein: Roles and Mechanisms

At the end of the multistep transcription process, the elongating RNA polymerase (RNAP) is dislodged from the DNA template either at specific DNA sequences, called the terminators, or by a nascent RNA-dependent helicase, Rho. In Escherichia coli, about half of the transcription events are terminated by the Rho protein. Rho utilizes its RNA-dependent ATPase activities to translocate along the mRNA and eventually dislodges the RNAP via an unknown mechanism. The transcription elongation factor NusG facilitates this termination process by directly interacting with Rho. In this review, we discuss current models describing the mechanism of action of this hexameric transcription terminator, its regulation by different cis and trans factors, and the effects of the termination process on physiological processes in bacterial cells, particularly E. coli and Salmonella enterica Typhimurium.

Location of the Substrate Binding Site of the Cytochrome bo3 Ubiquinol Oxidase from Escherichia coli

Cytochrome bo₃ is a respiratory proton-pumping oxygen reductase that is a member of the heme-copper superfamily that utilizes ubiquinol-8 (Q₈H₂) as a substrate. The current consensus model has Q₈H₂ oxidized at a low affinity site (Q_L), passing electrons to a tightly bound quinone cofactor at a high affinity site (Q_H site) that stabilizes the one-electron reduced ubisemiquinone, facilitating the transfer of electrons to the redox active metal centers where O₂ is reduced to water. The current work shows that the Q₈ bound to the Q_H site is more dynamic than previously thought. In addition, mutations of residues at the Q_H site that do not abolish activity have been re-examined and shown to have properties expected of mutations at the substrate binding site (Q_L): an increase in the K_M of the substrate ubiquinol-1 (up to 4-fold) and an increase in the apparent K_i of the inhibitor HQNO (up to 8-fold). The data suggest that there is only one binding site for ubiquinol in cyt bo₃ and that site corresponds to the Q_H site.

Molecular Basis of NusG-mediated Regulation of Rho-dependent Transcription Termination in Bacteria

The bacterial transcription elongation factor NusG stimulates the Rho-dependent transcription termination through a direct interaction with Rho. The mechanistic basis of NusG dependence of the Rho function is not known. Here, we describe Rho* mutants I168V, R221C/A, and P235H that do not require NusG for their termination function. These Rho* mutants have acquired new properties, which otherwise would have been imparted by NusG. A detailed analyses revealed that they have more stable interactions at the secondary RNA binding sites of Rho, which reduced the lag in initiating its ATPase as well as the translocase activities. These more stable interactions arose from the significant spatial re-orientations of the P, Q, and R structural loops of the Rho central channel. We propose that NusG imparts similar conformational changes in the central channel of Rho, yielding faster isomerization of the open to the closed hexameric states of the latter during its RNA-loading step. This acceleration stabilizes the Rho-RNA interactions at many terminators having suboptimal rut sites, thus making Rho-NusG interactions so essential in vivo Finally, identification of the NusG binding sites on the Rho hexamer led us to conclude that the former exerts its effect allosterically.

Transcription Elongation Factor NusA Is a General Antagonist of Rho-dependent Termination in Escherichia coli

NusA is an essential protein that binds to RNA polymerase and also to the nascent RNA and influences transcription by inducing pausing and facilitating the process of transcription termination/antitermination. Its participation in Rho-dependent transcription termination has been perceived, but the molecular nature of this involvement is not known. We hypothesized that, because both Rho and NusA are RNA-binding proteins and have the potential to target the same RNA, the latter is likely to influence the global pattern of the Rho-dependent termination. Analyses of the nascent RNA binding properties and consequent effects on the Rho-dependent termination functions of specific NusA-RNA binding domain mutants revealed an existence of Rho-NusA direct competition for the overlappingnut(NusA-binding site) andrut(Rho-binding site) sites on the RNA. This leads to delayed entry of Rho at therutsite that inhibits the latter's RNA release process. High density tiling microarray profiles of these NusA mutants revealed that a significant number of genes, together with transcripts from intergenic regions, are up-regulated. Interestingly, the majority of these genes were also up-regulated when the Rho function was compromised. These results provide strong evidence for the existence of NusA-binding sites in different operons that are also the targets of Rho-dependent terminations. Our data strongly argue in favor of a direct competition between NusA and Rho for the access of specific sites on the nascent transcripts in different parts of the genome. We propose that this competition enables NusA to function as a global antagonist of the Rho function, which is unlike its role as a facilitator of hairpin-dependent termination.

N protein from lambdoid phages transforms NusA into an antiterminator by modulating NusA-RNA polymerase flap domain interactions

Interaction of the lambdoid phage N protein with the bacterial transcription elongation factor NusA is the key component in the process of transcription antitermination. A convex surface of E. coli NusA-NTD, located opposite to its RNA polymerase-binding domain (the β-flap domain), directly interacts with N in the antitermination complex. We hypothesized that this N-NusA interaction induces allosteric effects on the NusA-RNAP interaction leading to transformation of NusA into a facilitator of the antitermination process. Here we showed that mutations in β-flap domain specifically defective for N antitermination exhibited altered NusA-nascent RNA interaction and have widened RNA exit channel indicating an intricate role of flap domain in the antitermination. The presence of N reoriented the RNAP binding surface of NusA-NTD, which changed its interaction pattern with the flap domain. These changes caused significant spatial rearrangement of the β-flap as well as the β′ dock domains to form a more constricted RNA exit channel in the N-modified elongation complex (EC), which might play key role in converting NusA into a facilitator of the N antitermination. We propose that in addition to affecting the RNA exit channel and the active center of the EC, β-flap domain rearrangement is also a mechanistic component in the N antitermination process.