An adapted method for Cas9-mediated editing reveals the species-specific role of β-glucoside utilization driving competition between Klebsiella species

Cas9-based gene editing tools have revolutionized genetics, enabling the fast and precise manipulation of diverse bacterial species. However, widely applicable genetic tools for non-model gut bacteria are unavailable. Here, we present a two-plasmid Cas9-based system designed for gene deletion and knock-in complementation in three members of the Klebsiella oxytoca species complex (KoSC), which we applied to study the genetic factors underlying the role of these bacteria in competition against Klebsiella pneumoniae. Firstly, the system allowed efficient and precise full-length gene deletion via enhanced lambda Red expression. Furthermore, we tested the efficiency of two independent, functionally validated complementation strategies. Ultimately, the insertion of universal "bookmark" targets during gene deletion subsequently allows the most optimal genetic complementation in K. oxytoca, Klebsiella michiganensis, and Klebsiella grimontii. This approach offers a significant advantage by enabling the use of a single high-efficiency "bookmark" for complementing other loci or strains, eliminating the need for site-specific design. We revealed that the carbohydrate permease CasA is critical in ex vivo assays for K. pneumoniae inhibition by K. oxytoca but is neither sufficient nor required for K. michiganensis and K. grimontii. Thus, the adaptation of state-of-the-art genetic tools to KoSC allows the identification of species-specific functions in microbial competition.

IMPORTANCE

Cas9-based gene editing tools have revolutionized bacterial genetics, yet, their application to non-model gut bacteria is frequently hampered by various limitations. We utilized a two-plasmid Cas9-based system designed for gene deletion in Klebsiella pneumoniae and demonstrate after optimization its utility for gene editing in three members of the Klebsiella oxytoca species complex (KoSC) namely K. oxytoca, Klebsiella michiganensis, and Klebsiella grimontii. We then adapted a recently developed protocol for functional complementation based on universal "bookmark" targets applicable to all tested species. In summary, species-specific adaptation of state-of-the-art genetic tools allows efficient gene deletion and complementation in type strains as well as natural isolates of KoSC members to study microbial interactions.

Global RNA interactome of Salmonella discovers a 5' UTR sponge for the MicF small RNA that connects membrane permeability to transport capacity

The envelope of Gram-negative bacteria is a vital barrier that must balance protection and nutrient uptake. Small RNAs are crucial regulators of the envelope composition and function. Here, using RIL-seq to capture the Hfq-mediated RNA-RNA interactome in Salmonella enterica, we discover envelope-related riboregulators, including OppX. We show that OppX acts as an RNA sponge of MicF sRNA, a prototypical porin repressor. OppX originates from the 5' UTR of oppABCDF, encoding the major inner-membrane oligopeptide transporter, and sequesters MicF's seed region to derepress the synthesis of the porin OmpF. Intriguingly, OppX operates as a true sponge, storing MicF in an inactive complex without affecting its levels or stability. Conservation of the opp-OppX-MicF-ompF axis in related bacteria suggests that it serves an important mechanism, adjusting envelope porosity to specific transport capacity. These data also highlight the resource value of this Salmonella RNA interactome, which will aid in unraveling RNA-centric regulation in enteric pathogens.

Scanning mutagenesis of RNA-binding protein ProQ reveals a quality control role for the Lon protease

The FinO-domain protein ProQ belongs to a widespread family of RNA-binding proteins (RBPs) involved in gene regulation in bacterial chromosomes and mobile elements. While the cellular RNA targets of ProQ have been established in diverse bacteria, the functionally crucial ProQ residues remain to be identified under physiological conditions. Following our discovery that ProQ deficiency alleviates growth suppression of Salmonella with succinate as the sole carbon source, an experimental evolution approach was devised to exploit this phenotype. By coupling mutational scanning with loss-of-function selection, we identified multiple ProQ residues in both the amino-terminal FinO domain and the variable carboxy-terminal region that are required for ProQ activity. Two carboxy-terminal mutations abrogated ProQ function and mildly impaired binding of a model RNA target. In contrast, several mutations in the FinO domain rendered ProQ both functionally inactive and unable to interact with target RNA in vivo. Alteration of the FinO domain stimulated the rapid turnover of ProQ by Lon-mediated proteolysis, suggesting a quality control mechanism that prevents the accumulation of nonfunctional ProQ molecules. We extend this observation to Hfq, the other major sRNA chaperone of enteric bacteria. The Hfq Y55A mutant protein, defective in RNA-binding and oligomerization, proved to be labile and susceptible to degradation by Lon. Taken together, our findings connect the major AAA+ family protease Lon with RNA-dependent quality control of Hfq and ProQ, the two major sRNA chaperones of Gram-negative bacteria.

In vivo targets of Salmonella FinO include a FinP-like small RNA controlling copy number of a cohabitating plasmid

FinO-domain proteins represent an emerging family of RNA-binding proteins (RBPs) with diverse roles in bacterial post-transcriptional control and physiology. They exhibit an intriguing targeting spectrum, ranging from an assumed single RNA pair (FinP/traJ) for the plasmid-encoded FinO protein, to transcriptome-wide activity as documented for chromosomally encoded ProQ proteins. Thus, the shared FinO domain might bear an unusual plasticity enabling it to act either selectively or promiscuously on the same cellular RNA pool. One caveat to this model is that the full suite of in vivo targets of the assumedly highly selective FinO protein is unknown. Here, we have extensively profiled cellular transcripts associated with the virulence plasmid-encoded FinO in Salmonella enterica. While our analysis confirms the FinP sRNA of plasmid pSLT as the primary FinO target, we identify a second major ligand: the RepX sRNA of the unrelated antibiotic resistance plasmid pRSF1010. FinP and RepX are strikingly similar in length and structure, but not in primary sequence, and so may provide clues to understanding the high selectivity of FinO-RNA interactions. Moreover, we observe that the FinO RBP encoded on the Salmonella virulence plasmid controls the replication of a cohabitating antibiotic resistance plasmid, suggesting cross-regulation of plasmids on the RNA level.

Differential Regulation of CsrC and CsrB by CRP-cAMP in Salmonella enterica

Post-transcriptional regulation mediated by regulatory small RNAs (sRNAs) has risen as a key player in fine-tuning gene expression in response to environmental stimuli. Here, we show that, in Salmonella enterica, the central metabolic regulator CRP-cAMP differentially regulates the sRNAs CsrB and CsrC in a growth phase-dependent manner. While CsrB expression remains unchanged during growth, CsrC displays a growth phase-dependent expression profile, being weakly expressed at the logarithmic growth phase and induced upon entry into stationary phase. We show that CRP-cAMP contributes to the expression pattern of CsrC by repressing its expression during the logarithmic growth phase. The CRP-cAMP mediated repression of CsrC is independent of SirA, a known transcriptional CsrB/CsrC activator. We further show that the sRNA Spot 42, which is derepressed in a Δcrp strain, upregulates CsrC during logarithmic growth. We propose a model where the growth-dependent regulation of CsrC is sustained by the CRP-cAMP-mediated repression of Spot 42. Together, our data point toward a differential regulation of the sRNAs CsrB and CsrC in response to environmental stimuli, leading to fine-tuning of gene expression via the sequestration of the RNA-binding protein CsrA.

Global discovery of bacterial RNA-binding proteins by RNase-sensitive gradient profiles reports a new FinO domain protein

RNA-binding proteins (RBPs) play important roles in bacterial gene expression and physiology but their true number and functional scope remain little understood even in model microbes. To advance global RBP discovery in bacteria, we here establish glycerol gradient sedimentation with RNase treatment and mass spectrometry (GradR). Applied to Salmonella enterica, GradR confirms many known RBPs such as CsrA, Hfq, and ProQ by their RNase-sensitive sedimentation profiles, and discovers the FopA protein as a new member of the emerging family of FinO/ProQ-like RBPs. FopA, encoded on resistance plasmid pCol1B9, primarily targets a small RNA associated with plasmid replication. The target suite of FopA dramatically differs from the related global RBP ProQ, revealing context-dependent selective RNA recognition by FinO-domain RBPs. Numerous other unexpected RNase-induced changes in gradient profiles suggest that cellular RNA helps to organize macromolecular complexes in bacteria. By enabling poly(A)-independent generic RBP discovery, GradR provides an important element in the quest to build a comprehensive catalog of microbial RBPs.

ppGpp mediates the growth phase-dependent regulation of agn43, a phase variable gene, by stimulating its promoter activity

Antigen 43 (Ag43) is a self-recognizing outer membrane protein of Escherichia coli expressed during intracellular growth and biofilm formation, suggesting a role in infection. The expression of agn43 is under phase variation control, meaning that there are regulatory mechanisms adjusting the percentage of agn43-expressing cells in the population, in addition to mechanisms modulating the transcriptional expression level in each expressing cell. Phenotypic and transcriptional studies indicate that Ag43 expression is induced upon entry into the stationary phase in a ppGpp-dependent and RpoS-independent manner. The use of single-cell approaches and phase variation deficient strains let to conclude that ppGpp stimulates agn43 promoter activity, rather than affecting the percentage of agn43-expressing cells. The data highlight the relevance that promoter activity regulation may have, without any involvement of the phase variation state, in the final Ag43 expression output. The agn43 promoter of the MG1655 strain carries an AT-rich discriminator between positions -10 and +1, which is highly conserved among the agn43 genes present in the different pathotypes of E. coli. Remarkably, the AT-rich discriminator is required for the positive transcriptional control mediated by ppGpp.

3'untranslated regions: regulation at the end of the road

Post-transcriptional gene regulation in bacteria plays a major role in the adaptation of bacterial cells to the changing conditions encountered in the environment. In bacteria, most of the regulation at the level of mRNA seems to be targeting the 5'untranslated regions where accessibility to the ribosome-binding site can be modulated to alter gene expression. In recent years, the role of 3'untranslated regions has gained attention also as a site for post-transcriptional regulation. In addition to be a source of trans-encoded small RNAs, the 3'untranslated regions can be targets to modulate gene expression. Taking recent findings in the post-transcriptional regulation of the hilD gene, encoding for the main regulator of virulence in Salmonella enterica serovar Typhimurium, we highlight the role of 3'untranslated regions as targets of post-transcriptional regulation mediated by small RNAs and discuss the implications of transcriptional elongation in the 3'UTR-mediated regulation in bacteria.

Gre factors-mediated control of hilD transcription is essential for the invasion of epithelial cells by Salmonella enterica serovar Typhimurium

The invasion of epithelial cells by Salmonella enterica serovar Typhimurium is a very tightly regulated process. Signaling cascades triggered by different environmental and physiological signals converge to control HilD, an AraC regulator that coordinates the expression of several virulence factors. The expression of hilD is modulated at several steps of the expression process. Here, we report that the invasion of epithelial cells by S. Typhimurium strains lacking the Gre factors, GreA and GreB, is impaired. By interacting with the RNA polymerase secondary channel, the Gre factors prevent backtracking of paused complexes to avoid arrest during transcriptional elongation. Our results indicate that the Gre factors are required for the expression of the bacterial factors needed for epithelial cell invasion by modulating expression of HilD. This regulation does not occur at transcription initiation and depends on the capacity of the Gre factors to prevent backtracking of the RNA polymerase. Remarkably, genetic analyses indicate that the 3’-untranslated region (UTR) of hilD is required for Gre-mediated regulation of hilD expression. Our data provide new insight into the complex regulation of S. Typhimurium virulence and highlight the role of the hilD 3’-UTR as a regulatory motif.

Salmonella enterica serovar Typhimurium is a foodborne pathogen that causes gastroenteritis in humans. To successfully trigger infection, S. Typhimurium invades epithelial cells, a process that requires the coordinated expression of a set of genes. HilD is a pivotal regulator of S. Typhimurium pathogenicity, as it activates the expression of the genes required for invasion of intestinal epithelium. Expression and activity of HilD are tightly regulated and respond to several environmental and physiological conditions. In this report, we introduce the transcription elongation as a novel level of regulation of hilD. We describe that the Gre factors, proteins that prevent backtracking of paused RNA polymerase complexes during transcription elongation, are required for the expression of HilD and the subsequent expression of genes involved in the invasion of epithelial cells.

The EAL-like protein STM1697 regulates virulence phenotypes, motility and biofilm formation in Salmonella typhimurium

The ubiquitous second messenger c-di-GMP regulates the switching of bacterial lifestyles from motility to sessility and acute to chronic virulence to adjust bacterial fitness to altered environmental conditions. Conventionally, EAL proteins being c-di-GMP phosphodiesterases promote motility and acute virulence phenotypes such as invasion into epithelial cells and inhibit biofilm formation. We report here that in contradiction, the EAL-like protein STM1697 of Salmonella typhimurium suppresses motility, invasion into HT-29 epithelial cell line and secretion of the type three secretion system 1 effector protein SipA, whereas it promotes rdar biofilm formation and CsgD expression. STM1697 can, however, functionally replace the EAL-like protein STM1344 and vice versa, whereby both proteins neither degrade nor bind c-di-GMP. Like STM1344, STM1697 suppresses the transcription of class 2 and class 3 flagella regulon genes by binding to FlhD, a component of the master regulator of the flagella regulon FlhD4 C2 and act additively under numerous conditions. Interestingly, the interaction interface of STM1697 with FlhD2 is distinct from its paralogue STM1344. We predict that the stand alone EAL domain proteins STM1697 and STM1344 belong to a subclass of EAL domain proteins in S. typhimurium, which are all involved in motility, biofilm and virulence regulation through interaction with proteins that regulate flagella function.