Selective ribosome profiling reveals a role for SecB in the co-translational inner membrane protein biogenesis.

The chaperone SecB has been implicated in de novo protein folding and translocation across the membrane, but it remains unclear which nascent polypeptides SecB binds, when during translation SecB acts, how SecB function is coordinated with other chaperones and targeting factors, and how polypeptide engagement contributes to protein biogenesis. Using selective ribosome profiling, we show that SecB binds many nascent cytoplasmic and translocated proteins generally late during translation and controlled by the chaperone trigger factor. Revealing an uncharted role in co-translational translocation, inner membrane proteins (IMPs) are the most prominent nascent SecB interactors. Unlike other substrates, IMPs are bound early during translation, following the membrane targeting by the signal recognition particle. SecB remains bound until translation is terminated, and contributes to membrane insertion. Our study establishes a role of SecB in the co-translational maturation of proteins from all cellular compartments and functionally implicates cytosolic chaperones in membrane protein biogenesis.

Mechanisms of Cotranslational Protein Maturation in Bacteria.

Growing cells invest a significant part of their biosynthetic capacity into the production of proteins. To become functional, newly-synthesized proteins must be N-terminally processed, folded and often translocated to other cellular compartments. A general strategy is to integrate these protein maturation processes with translation, by cotranslationally engaging processing enzymes, chaperones and targeting factors with the nascent polypeptide. Precise coordination of all factors involved is critical for the efficiency and accuracy of protein synthesis and cellular homeostasis. This review provides an overview of the current knowledge on cotranslational protein maturation, with a focus on the production of cytosolic proteins in bacteria. We describe the role of the ribosome and the chaperone network in protein folding and how the dynamic interplay of all cotranslationally acting factors guides the sequence of cotranslational events. Finally, we discuss recent data demonstrating the coupling of protein synthesis with the assembly of protein complexes and end with a brief discussion of outstanding questions and emerging concepts in the field of cotranslational protein maturation.

A colorimetric RT-LAMP assay and LAMP-sequencing for detecting SARS-CoV-2 RNA in clinical samples.

The coronavirus disease 2019 (COVID-19) pandemic caused by the SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2) coronavirus is a major public health challenge. Rapid tests for detecting existing SARS-CoV-2 infections and assessing virus spread are critical. Approaches to detect viral RNA based on reverse transcription loop-mediated isothermal amplification (RT-LAMP) have potential as simple, scalable, and broadly applicable testing methods. Compared to RT quantitative polymerase chain reaction (RT-qPCR)-based methods, RT-LAMP assays require incubation at a constant temperature, thus eliminating the need for sophisticated instrumentation. Here, we tested a two-color RT-LAMP assay protocol for detecting SARS-CoV-2 viral RNA using a primer set specific for the N gene. We tested our RT-LAMP assay on surplus RNA samples isolated from 768 pharyngeal swab specimens collected from individuals being tested for COVID-19. We determined the sensitivity and specificity of the RT-LAMP assay for detecting SARS-CoV-2 viral RNA. Compared to an RT-qPCR assay using a sensitive primer set, we found that the RT-LAMP assay reliably detected SARS-CoV-2 RNA with an RT-qPCR cycle threshold (CT) number of up to 30, with a sensitivity of 97.5% and a specificity of 99.7%. We also developed a swab-to-RT-LAMP assay that did not require a prior RNA isolation step, which retained excellent specificity (99.5%) but showed lower sensitivity (86% for CT < 30) than the RT-LAMP assay. In addition, we developed a multiplexed sequencing protocol (LAMP-sequencing) as a diagnostic validation procedure to detect and record the outcome of RT-LAMP reactions.

Selective ribosome profiling to study interactions of translating ribosomes in yeast.

A number of enzymes, targeting factors and chaperones engage ribosomes to support fundamental steps of nascent protein maturation, including enzymatic processing, membrane targeting and co-translational folding. The selective ribosome profiling (SeRP) method is a new tool for studying the co-translational activity of maturation factors that provides proteome-wide information on a factor's nascent interactome, the onset and duration of binding and the mechanisms controlling factor engagement. SeRP is based on the combination of two ribosome-profiling (RP) experiments, sequencing the ribosome-protected mRNA fragments from all ribosomes (total translatome) and the ribosome subpopulation engaged by the factor of interest (factor-bound translatome). We provide a detailed SeRP protocol, exemplified for the yeast Hsp70 chaperone Ssb (stress 70 B), for studying factor interactions with nascent proteins that is readily adaptable to identifying nascent interactomes of other co-translationally acting eukaryotic factors. The protocol provides general guidance for experimental design and optimization, as well as detailed instructions for cell growth and harvest, the isolation of (factor-engaged) monosomes, the generation of a cDNA library and data analysis. Experience in biochemistry and RNA handling, as well as basic programing knowledge, is necessary to perform SeRP. Execution of a SeRP experiment takes 8-10 working days, and initial data analysis can be completed within 1-2 d. This protocol is an extension of the originally developed protocol describing SeRP in bacteria.

Analyzing the Complex Regulatory Landscape of Hfq - an Integrative, Multi-Omics Approach.

The ability of bacteria to respond to environmental change is based on the ability to coordinate, redirect and fine-tune their genetic repertoire as and when required. While we can learn a great deal from reductive analysis of individual pathways and global approaches to gene regulation, a deeper understanding of these complex signaling networks requires the simultaneous consideration of several regulatory layers at the genome scale. To highlight the power of this approach we analyzed the Hfq transcriptional/translational regulatory network in the model bacterium Pseudomonas fluorescens. We first used extensive 'omics' analyses to assess how hfq deletion affects mRNA abundance, mRNA translation and protein abundance. The subsequent, multi-level integration of these datasets allows us to highlight the discrete contributions by Hfq to gene regulation at different levels. The integrative approach to regulatory analysis we describe here has significant potential, for both dissecting individual signaling pathways and understanding the strategies bacteria use to cope with external challenges.

Isolation of Ribosomal Particles from the Unicellular Cyanobacterium Synechocystis sp. PCC 6803.

Isolation of ribosomal particles is an essential step in the study of ribosomal components as well as in the analysis of trans-acting factors that interact with the ribosome to regulate protein synthesis and modulate the expression profile of the cell in response to different environmental conditions. In this protocol, we describe a procedure for the isolation of 70S ribosomes from the unicellular cyanobacterium Synechocystis sp. PCC 6803 (hereafter Synechocystis). We have successfully used this protocol in our study of the cyanobacterial ribosomal-associated protein LrtA, which is a homologue of bacterial HPF (hibernation promoting factor) ( Galmozzi et al., 2016 ).

NADPH-Thioredoxin Reductase C Mediates the Response to Oxidative Stress and Thermotolerance in the Cyanobacterium Anabaena sp. PCC7120.

NADPH-thioredoxin reductase C (NTRC) is a bimodular enzyme composed of an NADPH-thioredoxin reductase and a thiioredoxin domain extension in the same protein. In plants, NTRC has been described to be involved in the protection of the chloroplast against oxidative stress damage through reduction of the 2-Cys peroxiredoxin (2-Cys Prx) as well as through other functions related to redox enzyme regulation. In cyanobacteria, the Anabaena NTRC has been characterized in vitro, however, nothing was known about its in vivo function. In order to study that, we have generated the first knockout mutant strain (ΔntrC), apart from the previously described in Arabidopsis. Detailed characterization of this strain reveals a differential sensitivity to oxidative stress treatments with respect to the wild-type Anabaena strain, including a higher level of ROS (reactive oxygen species) in normal growth conditions. In the mutant strain, different oxidative stress treatments such as hydrogen peroxide, methyl-viologen or high light irradiance provoke an increase in the expression of genes related to ROS detoxification, including AnNTRC and peroxiredoxin genes, with a concomitant increase in the amount of AnNTRC and 2-Cys Prx. Moreover, the role of AnNTRC in the antioxidant response is confirmed by the observation of a pronounced overoxidation of the 2-Cys Prx and a time-delay recovery of the reduced form of this protein upon oxidative stress treatments. Our results suggest the participation of this enzyme in the peroxide detoxification in Anabaena. In addition, we describe the role of Anabaena NTRC in thermotolerance, by the appearance of high molecular mass AnNTRC complexes, showing that the mutant strain is more sensitive to high temperature treatments.

The Cyanobacterial Ribosomal-Associated Protein LrtA Is Involved in Post-Stress Survival in Synechocystis sp. PCC 6803.

A light-repressed transcript encodes the LrtA protein in cyanobacteria. We show that half-life of lrtA transcript from Synechocystis sp. PCC 6803 is higher in dark-treated cells as compared to light-grown cells, suggesting post-transcriptional control of lrtA expression. The lrtA 5´ untranslated leader region is involved in that darkness-dependent regulation. We also found that Synechocystis sp. PCC 6803 LrtA is a ribosome-associated protein present in both 30S and 70S ribosomal particles. In order to investigate the function of this protein we have constructed a deletion mutant of the lrtA gene. Cells lacking LrtA (∆lrtA) had significantly lower amount of 70S particles and a greater amount of 30S and 50S particles, suggesting a role of LrtA in stabilizing 70S particles. Synechocystis strains with different amounts of LrtA protein: wild-type, ∆lrtA, and LrtAS (overexpressing lrtA) showed no differences in their growth rate under standard laboratory conditions. However, a clear LrtA dose-dependent effect was observed in the presence of the antibiotic tylosin, being the LrtAS strains the most sensitive. Similar results were obtained under hyperosmotic stress caused by sorbitol. Conversely, after prolonged periods of starvation, ∆lrtA strains were delayed in their growth with respect to the wild-type and the LrtAS strains. A positive role of LrtA protein in post-stress survival is proposed.

NtcA is responsible for accumulation of the small isoform of ferredoxin:NADP oxidoreductase.

In several cyanobacteria, petH, the gene encoding ferredoxin:NADP oxidoreductase (FNR), is transcribed from at least two promoters depending on growth conditions. Two transcripts (short and long) are translated from two different translation initiation sites, resulting in two isoforms (large and small, respectively). Here, we show that in Synechocystis PCC6803 the global transcriptional regulator NtcA activates transcription from the distal petH promoter. Modification of the NtcA-binding site prevents NtcA binding to the promoter in vitro and abolishes accumulation of the small isoform of FNR in vivo. We also demonstrate that a similar petH transcription and translation regime occurs in other cyanobacteria. The conditions under which this system operates provide hints for the function of each FNR isoform.

The inactivating factor of glutamine synthetase IF17 is an intrinsically disordered protein, which folds upon binding to its target.

In cyanobacteria, ammonium is incorporated into carbon skeletons by the sequential action of glutamine synthetase and glutamate synthase (GOGAT). The activity of Synechocystis sp. PCC 6803 glutamine synthetase type I (GS) is controlled by a post-transcriptional process involving protein-protein interactions with two inactivating factors: the 65-residue-long protein (IF7) and the 149-residue-long one (IF17). The sequence of the C terminus of IF17 is similar to IF7; IF7 is an intrinsically disordered protein (IDP). In this work, we study the structural propensities and affinity for GS of IF17 and a chimera protein, IF17N/IF7 (constructed by fusing the first 82 residues of IF17 with the whole IF7) by fluorescence, CD, and NMR. IF17 and IF17N/IF7 are IDPs with residual non-hydrogen-bonded structure, probably formed by α-helical, turn-like, and PPII conformations; several theoretical predictions support these experimental findings. IF17 seems to fold upon binding to GS, as suggested by CD thermal denaturations and steady-state far-UV spectra. The apparent affinity of IF17 for GS, as measured by fluorescence, is slightly smaller (K(D) ~1 µM) than that measured for IF7 (~0.3 µM). The K(D)s determined by CD are similar to those measured by fluorescence, but slightly larger, suggesting possible conformational rearrangements in the IFs and/or GS upon binding. Further, the results with IFN17/IF7 suggest that (i) binding of IF17 to the GS is modulated not only by its C-terminal region but also by its N-terminus and (ii) there are weakly structured (that is, "fuzzy") complexes in the ternary GS-IF system.

Mutational analysis of the inactivating factors, IF7 and IF17 from Synechocystis sp. PCC 6803: critical role of arginine amino acid residues for glutamine synthetase inactivation.

The Synechocystis sp. PCC 6803 glutamine synthetase type I (GS) activity is controlled by a process that involves protein-protein interaction with two inactivating factors (IF7 and IF17). IF7 is a natively unfolded, 65-residue-long protein, homologous to the carboxy-terminal region of IF17. Both proteins have abundance of positively charged amino acid residues and a high isoelectric point. In this study, we analyse the IF amino acid residues involved in GS inactivation by a mutational approach, both in vitro and in vivo. The results clearly indicate that the GS-IF complex formation must be determined mainly by electrostatic interactions. We have identified three conserved arginine residues of IF7 and IF17 that are essential for the interaction of these proteins with GS. All these residues map in the homologous region of IFs. Furthermore, in vitro analysis of a truncated IF17 protein without the 82-residue-long amino-terminal part, together with the analysis of a Synechocystis strain expressing a chimeric protein, containing this amino-terminal part of IF17 fused to IF7, demonstrates that amino-terminal region of IF17 mostly confers a higher stability to this protein.

Posttranscriptional regulation of glutamine synthetase in the filamentous Cyanobacterium Anabaena sp. PCC 7120: differential expression between vegetative cells and heterocysts.

Genes homologous to those implicated in glutamine synthetase (GS) regulation by protein-protein interaction in the cyanobacterium Synechocystis sp. strain PCC 6803 are conserved in several cyanobacterial sequenced genomes. We investigated this GS regulatory mechanism in Anabaena sp. strain PCC 7120. In this strain the system operates with only one GS inactivation factor (inactivation factor 7A [IF7A]), encoded by open reading frame (ORF) asl2329 (gifA). Following addition of ammonium, expression of gifA is derepressed, leading to the synthesis of IF7A, and consequently, GS is inactivated. Upon ammonium removal, the GS activity returns to the initial level and IF7A becomes undetectable. The global nitrogen control protein NtcA binds to the gifA promoter. Constitutive high expression levels of gifA were found in an Anabaena ntcA mutant (CSE2), indicating a repressor role for NtcA. In vitro studies demonstrate that Anabaena GS is not inactivated by Synechocystis IFs (IF7 and IF17), indicating the specificity of the system. We constructed an Anabaena strain expressing a second inactivating factor, containing the amino-terminal part of IF17 from Synechocystis fused to IF7A. GS inactivation in this strain is more effective than that in the wild type (WT) and resembles that observed in Synechocystis. Finally we found differential expression of the IF system between heterocysts and vegetative cells of Anabaena.

The ammonium-inactivated cyanobacterial glutamine synthetase I is reactivated in vivo by a mechanism involving proteolytic removal of its inactivating factors.

The Synechocystis sp. PCC 6803 glutamine synthetase type I (GS) activity is controlled by a process that involves protein-protein interaction with two inactivating factors (IF7 and IF17). Following addition of ammonium, the genes encoding these proteins, gifA and gifB, respectively, are derepressed, leading to the synthesis of IF7 and IF17 and consequently GS is inactivated. Upon ammonium removal, the GS activity rapidly returns to the initial level within 20 min. In this study, we analyse the mechanism underlying GS reactivation and find that this process involves IF7 and IF17 degradation. We show that the presence of ammonium as nitrogen source enhances IF17 but not IF7 stability independently of gif gene transcription. Studies with Synechocystis crude extracts under different conditions revealed that IF7 and IF17 display different stabilities in vitro. We found that IF7 is degraded in vitro by the activity of metalloproteases. Furthermore, the involvement of soluble processing metallopeptidases in IF7 degradation has also been demonstrated in vivo, by analysing Synechocystis mutant strains devoid of genes of the prp family. Finally, using a Synechocystis strain lacking GS type I, we establish the crucial role of the target protein GS for in vivo IF7 and IF17 stability.