Inability of Prevotella bryantii to form a functional Shine-Dalgarno interaction reflects unique evolution of ribosome binding sites in Bacteroidetes

Cas9-assisted biological containment of a genetically engineered human commensal bacterium and genetic elements

Sophisticated gene circuits built by synthetic biology can enable bacteria to sense their environment and respond predictably. Engineered biosensing bacteria outfitted with such circuits can potentially probe the human gut microbiome to prevent, diagnose, or treat disease. To provide robust biocontainment for engineered bacteria, we devised a Cas9-assisted auxotrophic biocontainment system combining thymidine auxotrophy, an Engineered Riboregulator (ER) for controlled gene expression, and a CRISPR Device (CD). The CD prevents the engineered bacteria from acquiring thyA via horizontal gene transfer, which would disrupt the biocontainment system, and inhibits the spread of genetic elements by killing bacteria harboring the gene cassette. This system tunably controlled gene expression in the human gut commensal bacterium Bacteroides thetaiotaomicron, prevented escape from thymidine auxotrophy, and blocked transgene dissemination. These capabilities were validated in vitro and in vivo. This biocontainment system exemplifies a powerful strategy for bringing genetically engineered microorganisms safely into biomedicine.

Novel Signal Peptides and Episomal Plasmid System for Enhanced Protein Secretion in Engineered Bacteroides Species

The genus Bacteroides, a predominant group in the human gut microbiome, presents significant potential for microbiome engineering and the development of live biotherapeutics aimed at treating gut diseases. Despite its promising capabilities, tools for effectively engineering Bacteroides species have been limited. In our study, we have made a breakthrough by identifying novel signal peptides in Bacteroides thetaiotaomicron and Akkermansia muciniphila. These peptides facilitate efficient protein transport across cellular membranes in Bacteroides, a critical step for therapeutic applications. Additionally, we have developed an advanced episomal plasmid system. This system demonstrates superior protein secretion capabilities compared to traditional chromosomal integration plasmids, making it a vital tool for enhancing the delivery of therapeutic proteins in Bacteroides species. Initially, the stability of this episomal plasmid posed a challenge; however, we have overcome this by incorporating an essential gene-based selection system. This novel strategy not only ensures plasmid stability but also aligns with the growing need for antibiotic-free selection methods in clinical settings. Our work, therefore, not only provides a more robust secretion system for Bacteroides but also sets a new standard for the development of live biotherapeutics.

Analysis of programmed frameshifting during translation of prfB in Flavobacterium johnsoniae

Ribosomes of Bacteroidia fail to recognize Shine-Dalgarno (SD) sequences due to sequestration of the 3' tail of the 16S rRNA on the 30S platform. Yet in these organisms, the prfB gene typically contains the programmed +1 frameshift site with its characteristic SD sequence. Here, we investigate prfB autoregulation in Flavobacterium johnsoniae, a member of the Bacteroidia. We find that the efficiency of prfB frameshifting in F. johnsoniae is low (∼7%) relative to that in Escherichia coli (∼50%). Mutation or truncation of bS21 in F. johnsoniae increases frameshifting substantially, suggesting that anti-SD (ASD) sequestration is responsible for the reduced efficiency. The frameshift site of certain Flavobacteriales, such as Winogradskyella psychrotolerans, has no SD. In F. johnsoniae, this W. psychrotolerans sequence supports frameshifting as well as the native sequence, and mutation of bS21 causes no enhancement. These data suggest that prfB frameshifting normally occurs without SD-ASD pairing, at least under optimal laboratory growth conditions. Chromosomal mutations that remove the frameshift or ablate the SD confer subtle growth defects in the presence of paraquat or streptomycin, respectively, indicating that both the autoregulatory mechanism and the SD element contribute to F. johnsoniae cell fitness. Analysis of prfB frameshift sites across 2686 representative bacteria shows loss of the SD sequence in many clades, with no obvious relationship to genome-wide SD usage. These data reveal unexpected variation in the mechanism of frameshifting and identify another group of organisms, the Verrucomicrobiales, that globally lack SD sequences.

Unraveling the plasticity of translation initiation in prokaryotes: Beyond the invariant Shine-Dalgarno sequence

Translation initiation in prokaryotes is mainly defined, although not exclusively, by the interaction between the anti-Shine-Dalgarno sequence (antiSD), located at the 3'-terminus of the 16S ribosomal RNA, and a complementary sequence, the ribosome binding site, or Shine-Dalgarno (SD), located upstream of the start codon in prokaryotic mRNAs. The antiSD has a conserved 5'-CCUCC-3' core, but inter-species variations have been found regarding the participation of flanking bases in binding. These variations have been described for certain bacteria and, to a lesser extent, for some archaea. To further analyze these variations, we conducted binding-energy prediction analyses on over 6,400 genomic sequences from both domains. We identified 15 groups of antiSD variants that could be associated with the organisms' phylogenetic origin. Additionally, our findings revealed that certain organisms exhibit variations in the core itself. Importantly, an unaltered core is not necessarily required for the interaction between the 3'-terminus of the rRNA and the region preceding the AUG of the mRNA. In our study, we classified organisms into four distinct categories: i) those possessing a conserved core and demonstrating binding; ii) those with a conserved core but lacking evidence of binding; iii) those exhibiting binding in the absence of a conserved core; and iv) those lacking both a conserved core and evidence of binding. Our results demonstrate the flexibility of organisms in evolving different sequences involved in translation initiation beyond the traditional Shine-Dalgarno sequence. These findings are discussed in terms of the evolution of translation initiation in prokaryotic organisms.

The distinct translational landscapes of gram-negative Salmonella and gram-positive Listeria

Translational control in pathogenic bacteria is fundamental to gene expression and affects virulence and other infection phenotypes. We used an enhanced ribosome profiling protocol coupled with parallel transcriptomics to capture accurately the global translatome of two evolutionarily distant pathogenic bacteria-the Gram-negative bacterium Salmonella and the Gram-positive bacterium Listeria. We find that the two bacteria use different mechanisms to translationally regulate protein synthesis. In Salmonella, in addition to the expected correlation between translational efficiency and cis-regulatory features such as Shine-Dalgarno (SD) strength and RNA secondary structure around the initiation codon, our data reveal an effect of the 2nd and 3rd codons, where the presence of tandem lysine codons (AAA-AAA) enhances translation in both Salmonella and E. coli. Strikingly, none of these features are seen in efficiently translated Listeria transcripts. Instead, approximately 20% of efficiently translated Listeria genes exhibit 70 S footprints seven nt upstream of the authentic start codon, suggesting that these genes may be subject to a novel translational initiation mechanism. Our results show that SD strength is not a direct hallmark of translational efficiency in all bacteria. Instead, Listeria has evolved additional mechanisms to control gene expression level that are distinct from those utilised by Salmonella and E. coli.

Ribosomes lacking bS21 gain function to regulate protein synthesis in Flavobacterium johnsoniae

Ribosomes of Bacteroidia (formerly Bacteroidetes) fail to recognize Shine-Dalgarno (SD) sequences even though they harbor the anti-SD (ASD) of 16S rRNA. Inhibition of SD-ASD pairing is due to sequestration of the 3' tail of 16S rRNA in a pocket formed by bS21, bS18, and bS6 on the 30S platform. Interestingly, in many Flavobacteriales, the gene encoding bS21, rpsU, contains an extended SD sequence. In this work, we present genetic and biochemical evidence that bS21 synthesis in Flavobacterium johnsoniae is autoregulated via a subpopulation of ribosomes that specifically lack bS21. Mutation or depletion of bS21 in the cell increases translation of reporters with strong SD sequences, such as rpsU'-gfp, but has no effect on other reporters. Purified ribosomes lacking bS21 (or its C-terminal region) exhibit higher rates of initiation on rpsU mRNA and lower rates of initiation on other (SD-less) mRNAs than control ribosomes. The mechanism of autoregulation depends on extensive pairing between mRNA and 16S rRNA, and exceptionally strong SD sequences, with predicted pairing free energies of < -13 kcal/mol, are characteristic of rpsU across the Bacteroidota. This work uncovers a clear example of specialized ribosomes in bacteria.

RiboGrove: a database of full-length prokaryotic 16S rRNA genes derived from completely assembled genomes

16S rRNA gene is frequently used for the identification of prokaryotic organisms and for phylogeny inference. Several specialized public databases exist that contain complete and partial sequences of 16S rRNA genes. In this paper, we present RiboGrove: the first publicly available database that comprises only full-length sequences of 16S rRNA genes originating from completely assembled prokaryotic genomes deposited in RefSeq. Despite being strongly biased towards frequently sequenced genomes, RiboGrove is a useful complement to existing 16S rRNA resources and allows for analyses that would not be possible using amplicon-derived gene sequences. For instance, the absence of partial gene sequences in RiboGrove allowed us to make a summary of prokaryotic organisms, which lack core anti-Shine-Dalgarno sequence in their 16S rRNA genes. In this study, we describe the collected sequence data and present the results of exploratory data analysis of 16S rRNA gene sequences.

Genetic tools for the redirection of the central carbon flow towards the production of lactate in the human gut bacterium Phocaeicola (Bacteroides) vulgatus

Species of the genera Bacteroides and Phocaeicola play an important role in the human colon. The organisms contribute to the degradation of complex heteropolysaccharides to small chain fatty acids, which are in part utilized by the human body. Furthermore, these organisms are involved in the synthesis of vitamins and other bioactive compounds. Of special interest is Phocaeicola vulgatus, originally classified as a Bacteroides species, due to its abundance in the human intestinal tract and its ability to degrade many plant-derived heteropolysaccharides. We analyzed different tools for the genetic modification of this microorganism, with respect to homologous gene expression of the ldh gene encoding a D-lactate dehydrogenase (LDH). Therefore, the ldh gene was cloned into the integration vector pMM656 and the shuttle vector pG106 for homologous gene expression in P. vulgatus. We determined the ldh copy number, transcript abundance, and the enzyme activity of the wild type and the mutants. The strain containing the shuttle vector showed an approx. 1500-fold increase in the ldh transcript concentration and an enhanced LDH activity that was about 200-fold higher compared to the parental strain. Overall, the proportion of lactate in the general catabolic carbon flow increased from 2.9% (wild type) to 28.5% in the LDH-overproducing mutant. This approach is a proof of concept, verifying the genetic accessibility of P. vulgatus and could form the basis for targeted genetic optimization. KEY POINTS: • A lactate dehydrogenase was overexpressed in Phocaeicola (Bacteroides) vulgatus. • The ldh transcript abundance and the LDH activity increased sharply in the mutant. • The proportion of lactate in the catabolic carbon flow increased to about 30%.

Comparative Analysis of anti-Shine- Dalgarno Function in Flavobacterium johnsoniae and Escherichia coli

The anti-Shine-Dalgarno (ASD) sequence of 16S rRNA is highly conserved across Bacteria, and yet usage of Shine-Dalgarno (SD) sequences in mRNA varies dramatically, depending on the lineage. Here, we compared the effects of ASD mutagenesis in Escherichia coli, a Gammaproteobacteria which commonly employs SD sequences, and Flavobacterium johnsoniae, a Bacteroidia which rarely does. In E. coli, 30S subunits carrying any single substitution at positions 1,535–1,539 confer dominant negative phenotypes, whereas subunits with mutations at positions 1,540–1,542 are sufficient to support cell growth. These data suggest that CCUCC (1,535–1,539) represents the functional core of the element in E. coli. In F. johnsoniae, deletion of three ribosomal RNA (rrn) operons slowed growth substantially, a phenotype largely rescued by a plasmid-borne copy of the rrn operon. Using this complementation system, we found that subunits with single mutations at positions 1,535–1,537 are as active as control subunits, in sharp contrast to the E. coli results. Moreover, subunits with quadruple substitution or complete replacement of the ASD retain substantial, albeit reduced, activity. Sedimentation analysis revealed that these mutant subunits are overrepresented in the subunit fractions and underrepresented in polysome fractions, suggesting some defect in 30S biogenesis and/or translation initiation. Nonetheless, our collective data indicate that the ASD plays a much smaller role in F. johnsoniae than in E. coli, consistent with SD usage in the two organisms.

Elucidation of the Translation Initiation Factor Interaction Network of Haloferax volcanii Reveals Coupling of Transcription and Translation in Haloarchaea

Translation is an important step in gene expression. Initiation of translation is rate-limiting, and it is phylogenetically more diverse than elongation or termination. Bacteria contain only three initiation factors. In stark contrast, eukaryotes contain more than 10 (subunits of) initiation factors (eIFs). The genomes of archaea contain many genes that are annotated to encode archaeal homologs of eukaryotic initiation factors (aIFs). However, experimental characterization of aIFs is scarce and mostly restricted to very few species. To broaden the view, the protein-protein interaction network of aIFs in the halophilic archaeon Haloferax volcanii has been characterized. To this end, tagged versions of 14 aIFs were overproduced, affinity isolated, and the co-isolated binding partners were identified by peptide mass fingerprinting and MS/MS analyses. The aIF-aIF interaction network was resolved, and it was found to contain two interaction hubs, (1) the universally conserved factor aIF5B, and (2) a protein that has been annotated as the enzyme ribose-1,5-bisphosphate isomerase, which we propose to rename to aIF2Bα. Affinity isolation of aIFs also led to the co-isolation of many ribosomal proteins, but also transcription factors and subunits of the RNA polymerase (Rpo). To analyze a possible coupling of transcription and translation, seven tagged Rpo subunits were overproduced, affinity isolated, and co-isolated proteins were identified. The Rpo interaction network contained many transcription factors, but also many ribosomal proteins as well as the initiation factors aIF5B and aIF2Bα. These results showed that transcription and translation are coupled in haloarchaea, like in Escherichia coli. It seems that aIF5B and aIF2Bα are not only interaction hubs in the translation initiation network, but also key players in the transcription-translation coupling.

Streamlined CRISPR genome engineering in wild-type bacteria using SIBR-Cas

CRISPR-Cas is a powerful tool for genome editing in bacteria. However, its efficacy is dependent on host factors (such as DNA repair pathways) and/or exogenous expression of recombinases. In this study, we mitigated these constraints by developing a simple and widely applicable genome engineering tool for bacteria which we termed SIBR-Cas (Self-splicing Intron-Based Riboswitch-Cas). SIBR-Cas was generated from a mutant library of the theophylline-dependent self-splicing T4 td intron that allows for tight and inducible control over CRISPR-Cas counter-selection. This control delays CRISPR-Cas counter-selection, granting more time for the editing event (e.g. by homologous recombination) to occur. Without the use of exogenous recombinases, SIBR-Cas was successfully applied to knock-out several genes in three wild-type bacteria species (Escherichia coli MG1655, Pseudomonas putida KT2440 and Flavobacterium IR1) with poor homologous recombination systems. Compared to other genome engineering tools, SIBR-Cas is simple, tightly regulated and widely applicable for most (non-model) bacteria. Furthermore, we propose that SIBR can have a wider application as a simple gene expression and gene regulation control mechanism for any gene or RNA of interest in bacteria.

Structural basis of sequestration of the anti-Shine-Dalgarno sequence in the Bacteroidetes ribosome

Genomic studies have indicated that certain bacterial lineages such as the Bacteroidetes lack Shine-Dalgarno (SD) sequences, and yet with few exceptions ribosomes of these organisms carry the canonical anti-SD (ASD) sequence. Here, we show that ribosomes purified from Flavobacterium johnsoniae, a representative of the Bacteroidetes, fail to recognize the SD sequence of mRNA in vitro. A cryo-electron microscopy structure of the complete 70S ribosome from F. johnsoniae at 2.8 Å resolution reveals that the ASD is sequestered by ribosomal proteins bS21, bS18 and bS6, explaining the basis of ASD inhibition. The structure also uncovers a novel ribosomal protein—bL38. Remarkably, in F. johnsoniae and many other Flavobacteriia, the gene encoding bS21 contains a strong SD, unlike virtually all other genes. A subset of Flavobacteriia have an alternative ASD, and in these organisms the fully complementary sequence lies upstream of the bS21 gene, indicative of natural covariation. In other Bacteroidetes classes, strong SDs are frequently found upstream of the genes for bS21 and/or bS18. We propose that these SDs are used as regulatory elements, enabling bS21 and bS18 to translationally control their own production.

Non-oral Prevotella stepping into the spotlight

Species now affiliated to genus Prevotella have been known for decades as an integral part of human oral cavity microbiota. They were frequently isolated from patients with periodontitis or from dental root canals but also from healthy subjects. With the exception of Prevotella intermedia, they were considered opportunistic pathogens, as they were isolated also from various bacterial abscesses from the head, neck, breast, skin and various other body sites. Consequently, Prevotella were not in the focus of research activities. On the other hand, the four species found in the rumen never caused any disease and seemed early on to be numerous and important part of the rumen ecosystem indicating this genus harbored bacteria with enormously diverse habitats and lifestyles. The purpose of this review is to illustrate the main research themes performed in Prevotella on a path from less noted oral bacteria and from hard to cultivate and study rumen organisms to important mutualistic bacteria in guts of various mammals warranting major research efforts.

The diversity of Shine-Dalgarno sequences sheds light on the evolution of translation initiation

Shine-Dalgarno (SD) sequences, the core element of prokaryotic ribosome-binding sites, facilitate mRNA translation by base-pair interaction with the anti-SD (aSD) sequence of 16S rRNA. In contrast to this paradigm, an inspection of thousands of prokaryotic species unravels tremendous SD sequence diversity both within and between genomes, whereas aSD sequences remain largely static. The pattern has led many to suggest unidentified mechanisms for translation initiation. Here we review known translation-initiation pathways in prokaryotes. Moreover, we seek to understand the cause and consequence of SD diversity through surveying recent advances in biochemistry, genomics, and high-throughput genetics. These findings collectively show: (1) SD:aSD base pairing is beneficial but nonessential to translation initiation. (2) The 5' untranslated region of mRNA evolves dynamically and correlates with organismal phylogeny and ecological niches. (3) Ribosomes have evolved distinct usage of translation-initiation pathways in different species. We propose a model portraying the SD diversity shaped by optimization of gene expression, adaptation to environments and growth demands, and the species-specific prerequisite of ribosomes to initiate translation. The model highlights the coevolution of ribosomes and mRNA features, leading to functional customization of the translation apparatus in each organism.

How Many Messenger RNAs Can Be Translated by the START Mechanism?

Translation initiation is a key step in the protein synthesis stage of the gene expression pathway of all living cells. In this important process, ribosomes have to accurately find the AUG start codon in order to ensure the integrity of the proteome. “Structure Assisted RNA Translation”, or “START”, has been proposed to use stable secondary structures located in the coding sequence to augment start site selection by steric hindrance of the progression of pre-initiation complex on messenger RNA. This implies that such structures have to be located downstream and at on optimal distance from the AUG start codon (i.e., downstream nucleotide +16). In order to assess the importance of the START mechanism in the overall mRNA translation process, we developed a bioinformatic tool to screen coding sequences for such stable structures in a 50 nucleotide-long window spanning the nucleotides from +16 to +65. We screened eight bacterial genomes and six eukaryotic genomes. We found stable structures in 0.6–2.5% of eukaryotic coding sequences. Among these, approximately half of them were structures predicted to form G-quadruplex structures. In humans, we selected 747 structures. In bacteria, the coding sequences from Gram-positive bacteria contained 2.6–4.2% stable structures, whereas the structures were less abundant in Gram-negative bacteria (0.2–2.7%). In contrast to eukaryotes, putative G-quadruplex structures are very rare in the coding sequence of bacteria. Altogether, our study reveals that the START mechanism seems to be an ancient strategy to facilitate the start codon recognition that is used in different kingdoms of life.

Global analysis of protein synthesis in Flavobacterium johnsoniae reveals the use of Kozak-like sequences in diverse bacteria

In all cells, initiation of translation is tuned by intrinsic features of the mRNA. Here, we analyze translation in Flavobacterium johnsoniae, a representative of the Bacteroidetes. Members of this phylum naturally lack Shine-Dalgarno (SD) sequences in their mRNA, and yet their ribosomes retain the conserved anti-SD sequence. Translation initiation is tuned by mRNA secondary structure and by the identities of several key nucleotides upstream of the start codon. Positive determinants include adenine at position -3, reminiscent of the Kozak sequence of Eukarya. Comparative analysis of Escherichia coli reveals use of the same Kozak-like sequence to enhance initiation, suggesting an ancient and widespread mechanism. Elimination of contacts between A-3 and the conserved β-hairpin of ribosomal protein uS7 fails to diminish the contribution of A-3 to initiation, suggesting an indirect mode of recognition. Also, we find that, in the Bacteroidetes, the trinucleotide AUG is underrepresented in the vicinity of the start codon, which presumably helps compensate for the absence of SD sequences in these organisms.

Emerging Frontiers in Microbiome Engineering

The gut microbiome has a significant impact on health and disease and can actively contribute to obesity, diabetes, inflammatory bowel disease, cardiovascular disease, and neurological disorders. We do not yet have the necessary tools to fine-tune the microbial communities that constitute the microbiome, though such tools could unlock extensive benefits to human health. Here, we provide an overview of the current state of technological tools that may be used for microbiome engineering. These tools can enable investigators to define the parameters of a healthy microbiome and to determine how gut bacteria may contribute to the etiology of a variety of diseases. These tools may also allow us to explore the exciting prospect of developing targeted therapies and personalized treatments for microbiome-linked diseases.

Translational coupling via termination-reinitiation in archaea and bacteria

The genomes of many prokaryotes contain substantial fractions of gene pairs with overlapping stop and start codons (ATGA or TGATG). A potential benefit of overlapping gene pairs is translational coupling. In 720 genomes of archaea and bacteria representing all major phyla, we identify substantial, albeit highly variable, fractions of co-directed overlapping gene pairs. Various patterns are observed for the utilization of the SD motif for de novo initiation at upstream genes versus reinitiation at overlapping gene pairs. We experimentally test the predicted coupling in 9 gene pairs from the archaeon Haloferax volcanii and 5 gene pairs from the bacterium Escherichia coli. In 13 of 14 cases, translation of both genes is strictly coupled. Mutational analysis of SD motifs located upstream of the downstream genes indicate that the contribution of the SD to translational coupling widely varies from gene to gene. The nearly universal, abundant occurrence of overlapping gene pairs suggests that tight translational coupling is widespread in archaea and bacteria.

Archaea and bacteria often have gene pairs with overlapping stop and start codons, suggesting translational coupling. Here, Huber et al. analyse overlapping gene pairs from 720 genomes, and validate translational coupling via termination-reinitiation for 14 gene pairs in Haloferax volcanii and Escherichia coli.

Genes within Genes in Bacterial Genomes

Genetic coding in bacteria largely operates via the "one gene-one protein" paradigm. However, the peculiarities of the mRNA structure, the versatility of the genetic code, and the dynamic nature of translation sometimes allow organisms to deviate from the standard rules of protein encoding. Bacteria can use several unorthodox modes of translation to express more than one protein from a single mRNA cistron. One such alternative path is the use of additional translation initiation sites within the gene. Proteins whose translation is initiated at different start sites within the same reading frame will differ in their N termini but will have identical C-terminal segments. On the other hand, alternative initiation of translation in a register different from the frame dictated by the primary start codon will yield a protein whose sequence is entirely different from the one encoded in the main frame. The use of internal mRNA codons as translation start sites is controlled by the nucleotide sequence and the mRNA folding. The proteins of the alternative proteome generated via the "genes-within-genes" strategy may carry important functions. In this review, we summarize the currently known examples of bacterial genes encoding more than one protein due to the utilization of additional translation start sites and discuss the known or proposed functions of the alternative polypeptides in relation to the main protein product of the gene. We also discuss recent proteome- and genome-wide approaches that will allow the discovery of novel translation initiation sites in a systematic fashion.

The diverse and extensive plant polysaccharide degradative apparatuses of the rumen and hindgut Prevotella species: A factor in their ubiquity?

Although the Prevotella are commonly observed in high shares in the mammalian hindgut and rumen studies using NGS approach, the knowledge on their actual role, though postulated to lie in soluble fibre degradation, is scarce. Here we analyse in total 23, more than threefold of hitherto known rumen and hindgut Prevotella species and show that rumen/hindgut Prevotella generally possess extensive repertoires of polysaccharide utilization loci (PULs) and carbohydrate active enzymes targeting various plant polysaccharides. These PUL repertoires separate analysed Prevotella into generalists and specialists yet a finer diversity among generalists is evident too, both in range of substrates targeted and in PUL combinations targeting the same broad substrate classes. Upon evaluation of the shares of species analysed in this study in rumen metagenomes we found firstly, that they contributed significantly to total Prevotella abundance though much of rumen Prevotella diversity may still be unknown. Secondly, the hindgut Prevotella species originally isolated in pigs and humans occasionally dominated among the Prevotella with surprisingly high metagenome read shares and were consistently found in rumen metagenome samples from sites as apart as New Zealand and Scotland. This may indicate frequent passage between different hosts and relatively low barriers to their successful establishment in rumen versus the hindgut.

Non-canonical Escherichia coli transcripts lacking a Shine-Dalgarno motif have very different translational efficiencies and do not form a coherent group

Translation initiation in 50-70 % of transcripts in Escherichia coli requires base pairing between the Shine-Dalgarno (SD) motif in the mRNA and the anti-SD motif at the 3' end of the 16S rRNA. However, 30-50 % of E. coli transcripts are non-canonical and are not preceded by an SD motif. The 5' ends of 44 E. coli transcripts were determined, all of which contained a 5'-UTR (no leaderless transcripts), but only a minority contained an SD motif. The 5'-UTR lengths were compared with those listed in RegulonDB and reported in previous publications, and the identities and differences were obtained in all possible combinations. We aimed to quantify the translational efficiencies of non-canonical 5'-UTRs using GusA reporter gene assays and Northern blot analyses. Ten non-canonical 5'-UTRs and two control 5'-UTRs with an SD motif were cloned upstream of the gusA gene. The translational efficiencies were quantified under five different conditions (different growth rates via two different temperatures and two different carbon sources, and heat shock). The translational efficiencies of the non-canonical 5'-UTRs varied widely, from 5 to 384 % of the positive control. In addition, the non-canonical transcripts did not exhibit a common regulatory pattern with changing environmental parameters. No correlation could be observed between the translational efficiencies of the non-canonical 5'-UTRs and their lengths, sequences, GC content, or predicted secondary structures. The introduction of an SD motif enhanced the translational efficiency of a poorly translated non-canonical transcript, while the efficiency of a well-translated non-canonical transcript remained unchanged. Taken together, the mechanisms of translation initiation at non-canonical transcripts in E. coli still need to be elucidated.

Comparative genomic analysis of translation initiation mechanisms for genes lacking the Shine-Dalgarno sequence in prokaryotes

In prokaryotes, translation initiation is believed to occur through an interaction between the 3΄ tail of a 16S rRNA and a corresponding Shine–Dalgarno (SD) sequence in the 5΄ untranslated region (UTR) of an mRNA. However, some genes lack SD sequences (non-SD genes), and the fraction of non-SD genes in a genome varies depending on the prokaryotic species. To elucidate non-SD translation initiation mechanisms in prokaryotes from an evolutionary perspective, we statistically examined the nucleotide frequencies around the initiation codons in non-SD genes from 260 prokaryotes (235 bacteria and 25 archaea). We identified distinct nucleotide frequency biases upstream of the initiation codon in bacteria and archaea, likely because of the presence of leaderless mRNAs lacking a 5΄ UTR. Moreover, we observed overall similarities in the nucleotide patterns between upstream and downstream regions of the initiation codon in all examined phyla. Symmetric nucleotide frequency biases might facilitate translation initiation by preventing the formation of secondary structures around the initiation codon. These features are more prominent in species’ genomes that harbor large fractions of non-SD sequences, suggesting that a reduced stability around the initiation codon is important for efficient translation initiation in prokaryotes.

Noncanonical Translation Initiation Comes of Age

The canonical translation initiation mechanism involves base pairing between the mRNA and 16S rRNA. However, a variety of identified mechanisms deviate from this conventional route. Beck and Janssen (J Bacteriol 199:e00091-17, 2017, https://doi.org/10.1128/JB.00091-17) have recently described another noncanonical mode of translation initiation. Here, we describe how this process differs from previously reported mechanisms, with the hope that it will foster increased awareness of the diversity of regulatory mechanisms that await discovery.

Engineered Regulatory Systems Modulate Gene Expression of Human Commensals in the Gut

The gut microbiota is implicated in numerous aspects of health and disease, but dissecting these connections is challenging because genetic tools for gut anaerobes are limited. Inducible promoters are particularly valuable tools because these platforms allow real-time analysis of the contribution of microbiome gene products to community assembly, host physiology, and disease. We developed a panel of tunable expression platforms for the prominent genus Bacteroides in which gene expression is controlled by a synthetic inducer. In the absence of inducer, promoter activity is fully repressed; addition of inducer rapidly increases gene expression by four to five orders of magnitude. Because the inducer is absent in mice and their diets, Bacteroides gene expression inside the gut can be modulated by providing the inducer in drinking water. We use this system to measure the dynamic relationship between commensal sialidase activity and liberation of mucosal sialic acid, a receptor and nutrient for pathogens. VIDEO ABSTRACT.

The Frequency of Internal Shine-Dalgarno-like Motifs in Prokaryotes

In prokaryotes, translation initiation typically depends on complementary binding between a G-rich Shine–Dalgarno (SD) motif in the 5′ untranslated region of mRNAs, and the 3′ tail of the 16S ribosomal RNA (the anti-SD sequence). In some cases, internal SD-like motifs in the coding region generate “programmed” ribosomal pauses that are beneficial for protein folding or accurate targeting. On the other hand, such pauses can also reduce protein production, generating purifying selection against internal SD-like motifs. This selection should be stronger in GC-rich genomes that are more likely to harbor the G-rich SD motif. However, the nature and consequences of selection acting on internal SD-like motifs within genomes and across species remains unclear. We analyzed the frequency of SD-like hexamers in the coding regions of 284 prokaryotes (277 with known anti-SD sequences and 7 without a typical SD mechanism). After accounting for GC content, we found that internal SD-like hexamers are avoided in 230 species, including three without a typical SD mechanism. The degree of avoidance was higher in GC-rich genomes, mesophiles, and N-terminal regions of genes. In contrast, 54 species either showed no signature of avoidance or were enriched in internal SD-like motifs. C-terminal gene regions were relatively enriched in SD-like hexamers, particularly for genes in operons or those followed closely by downstream genes. Together, our results suggest that the frequency of internal SD-like hexamers is governed by multiple factors including GC content and genome organization, and further empirical work is necessary to understand the evolution and functional roles of these motifs.

Polysaccharide utilization locus and CAZYme genome repertoires reveal diverse ecological adaptation of Prevotella species

The results of metagenomic studies have clearly established that bacteria of the genus Prevotella represent one of the important groups found in the oral cavity and large intestine of man, and they also dominate the rumen. They belong to the Bacteroidetes, a phylum well-known for its polysaccharide degrading potential that stems from the outer membrane-localized enzyme/binding protein complexes encoded in polysaccharide utilization loci (PULs). Dozens of Prevotella species have been described, primarily from the oral cavity, and many of them occur simultaneously at the same sites, but research on their ecological adaptation has been neglected. Therefore, in this study, the repertoires of PULs and carbohydrate acting enzymes (CAZYmes) found in Prevotella genomes were analyzed and it was concluded that the Prevotella species were widely heterogeneous in this respect and displayed several distinct adaptations with regard to the number, source and nature of the substrates apparently preferred for growth.

Initiation of mRNA translation in bacteria: structural and dynamic aspects

Initiation of mRNA translation is a major checkpoint for regulating level and fidelity of protein synthesis. Being rate limiting in protein synthesis, translation initiation also represents the target of many post-transcriptional mechanisms regulating gene expression. The process begins with the formation of an unstable 30S pre-initiation complex (30S pre-IC) containing initiation factors (IFs) IF1, IF2 and IF3, the translation initiation region of an mRNA and initiator fMet-tRNA whose codon and anticodon pair in the P-site following a first-order rearrangement of the 30S pre-IC produces a locked 30S initiation complex (30SIC); this is docked by the 50S subunit to form a 70S complex that, following several conformational changes, positional readjustments of its ligands and ejection of the IFs, becomes a 70S initiation complex productive in initiation dipeptide formation. The first EF-G-dependent translocation marks the beginning of the elongation phase of translation. Here, we review structural, mechanistic and dynamical aspects of this process.

Programming a Human Commensal Bacterium, Bacteroides thetaiotaomicron, to Sense and Respond to Stimuli in the Murine Gut Microbiota

Engineering commensal organisms for challenging applications, such as modulating the gut ecosystem, is hampered by the lack of genetic parts. Here, we describe promoters, ribosome-binding sites, and inducible systems for use in the commensal bacterium Bacteroides thetaiotaomicron, a prevalent and stable resident of the human gut. We achieve up to 10,000-fold range in constitutive gene expression and 100-fold regulation of gene expression with inducible promoters and use these parts to record DNA-encoded memory in the genome. We use CRISPR interference (CRISPRi) for regulated knockdown of recombinant and endogenous gene expression to alter the metabolic capacity of B. thetaiotaomicron and its resistance to antimicrobial peptides. Finally, we show that inducible CRISPRi and recombinase systems can function in B. thetaiotaomicron colonizing the mouse gut. These results provide a blueprint for engineering new chassis and a resource to engineer Bacteroides for surveillance of or therapeutic delivery to the gut microbiome.

KLIKK proteases of Tannerella forsythia: putative virulence factors with a unique domain structure

Comparative genomics of virulent Tannerella forsythia ATCC 43037 and a close health-associated relative, Tannerella BU063, revealed, in the latter, the absence of an entire array of genes encoding putative secretory proteases that possess a nearly identical C-terminal domain (CTD) that ends with a -Lys-Leu-Ile-Lys-Lys motif. This observation suggests that these proteins, referred to as KLIKK proteases, may function as virulence factors. Re-sequencing of the loci of the KLIKK proteases found only six genes grouped in two clusters. All six genes were expressed by T. forsythia in routine culture conditions, although at different levels. More importantly, a transcript of each gene was detected in gingival crevicular fluid (GCF) from periodontitis sites infected with T. forsythia indicating that the proteases are expressed in vivo. In each protein, a protease domain was flanked by a unique N-terminal profragment and a C-terminal extension ending with the CTD. Partially purified recombinant proteases showed variable levels of proteolytic activity in zymography gels and toward protein substrates, including collagen, gelatin, elastin, and casein. Taken together, these results indicate that the pathogenic strain of T. forsythia secretes active proteases capable of degrading an array of host proteins, which likely represents an important pathogenic feature of this bacterium.

The conserved GTPase LepA contributes mainly to translation initiation in Escherichia coli

LepA is a paralog of EF-G found in all bacteria. Deletion of lepA confers no obvious growth defect in Escherichia coli, and the physiological role of LepA remains unknown. Here, we identify nine strains (ΔdksA, ΔmolR1, ΔrsgA, ΔtatB, ΔtonB, ΔtolR, ΔubiF, ΔubiG or ΔubiH) in which ΔlepA confers a synthetic growth phenotype. These strains are compromised for gene regulation, ribosome assembly, transport and/or respiration, indicating that LepA contributes to these functions in some way. We also use ribosome profiling to deduce the effects of LepA on translation. We find that loss of LepA alters the average ribosome density (ARD) for hundreds of mRNA coding regions in the cell, substantially reducing ARD in many cases. By contrast, only subtle and codon-specific changes in ribosome distribution along mRNA are seen. These data suggest that LepA contributes mainly to the initiation phase of translation. Consistent with this interpretation, the effect of LepA on ARD is related to the sequence of the Shine–Dalgarno region. Global perturbation of gene expression in the ΔlepA mutant likely explains most of its phenotypes.

Haloferax volcanii, a prokaryotic species that does not use the Shine Dalgarno mechanism for translation initiation at 5'-UTRs

It was long assumed that translation initiation in prokaryotes generally occurs via the so-called Shine Dalgarno (SD) mechanism. Recently, it became clear that translation initiation in prokaryotes is more heterogeneous. In the haloarchaeon Haloferax volcanii, the majority of transcripts is leaderless and most transcripts with a 5′-UTR lack a SD motif. Nevertheless, a bioinformatic analysis predicted that 20–30% of all genes are preceded by a SD motif in haloarchaea. To analyze the importance of the SD mechanism for translation initiation in haloarchaea experimentally the monocistronic sod gene was chosen, which contains a 5′-UTR with an extensive SD motif of seven nucleotides and a length of 19 nt, the average length of 5′UTRs in this organism. A translational fusion of part of the sod gene with the dhfr reporter gene was constructed. A mutant series was generated that matched the SD motif from zero to eight positions, respectively. Surprisingly, there was no correlation between the base pairing ability between transcripts and 16S rRNA and translational efficiency in vivo under several different growth conditions. Furthermore, complete replacement of the SD motif by three unrelated sequences did not reduce translational efficiency. The results indicate that H. volcanii does not make use of the SD mechanism for translation initiation in 5′-UTRs. A genome analysis revealed that while the number of SD motifs in 5′-UTRs is rare, their fraction within open reading frames is high. Possible biological functions for intragenic SD motifs are discussed, including re-initiation of translation at distal genes in operons.

Defining the bacteroides ribosomal binding site

The human gastrointestinal tract, in particular the colon, hosts a vast number of commensal microorganisms. Representatives of the genus Bacteroides are among the most abundant bacterial species in the human colon. Bacteroidetes diverged from the common line of eubacterial descent before other eubacterial groups. As a result, they employ unique transcription initiation signals and, because of this uniqueness, they require specific genetic tools. Although some tools exist, they are not optimal for studying the roles and functions of these bacteria in the human gastrointestinal tract. Focusing on translation initiation signals in Bacteroides, we created a series of expression vectors allowing for different levels of protein expression in this genus, and we describe the use of pepI from Lactobacillus delbrueckii subsp. lactis as a novel reporter gene for Bacteroides. Furthermore, we report the identification of the 3' end of the 16S rRNA of Bacteroides ovatus and analyze in detail its ribosomal binding site, thus defining a core region necessary for efficient translation, which we have incorporated into the design of our expression vectors. Based on the sequence logo information from the 5' untranslated region of other Bacteroidales ribosomal protein genes, we conclude that our findings are relevant to all members of this order.

Bioinformatic evidence and characterization of novel putative large conjugative transposons residing in genomes of genera Bacteroides and Prevotella

Bioinformatic evidence of the presence of a large conjugative transposon in ruminal bacterium Prevotella bryantii B(1)4(T) is presented. The described transposon appears to be related to another large conjugative transposon CTnBST, described in Bacteroides uniformis WH207 and to the conjugative transposon CTn3-Bf, which was observed in the genome of Bacteroides fragilis strain YCH46. All three transposons share tra gene regions with high amino acid identity and clearly conserved gene order. Additionally, a second conserved region consisting of hypothetical genes was discovered in all three transposons and named the GG region. This region served as a specific sequence signature and made possible the discovery of several other apparently related hypothetical conjugative transposons in bacteria from the genus Bacteroides. A cluster of genes involved in sugar utilization and metabolism was discovered within the hypothetical CTnB(1)4, to a certain extent resembling the polysaccharide utilization loci which were described recently in some Bacteroides strains. This is the first firm report on the presence of a large mobile genetic element in any strain from the genus Prevotella.