Reciprocal regulation of Yersinia pestis biofilm formation and virulence by RovM and RovA

LysR-type transcriptional regulators: state of the art

The LysR-type transcriptional regulators (LTTRs) are DNA-binding proteins present in bacteria, archaea, and in algae. Knowledge about their distribution, abundance, evolution, structural organization, transcriptional regulation, fundamental roles in free life, pathogenesis, and bacteria-plant interaction has been generated. This review focuses on these aspects and provides a current picture of LTTR biology.

Comparative Lysine Acetylome Analysis of Y. pestis YfiQ/CobB Mutants Reveals that Acetylation of SlyA Lys73 Significantly Promotes Biofilm Formation of Y. pestis

Increasing evidence shows that protein lysine acetylation is involved in almost every aspect of cellular physiology in bacteria. Yersinia pestis is a flea-borne pathogen responsible for millions of human deaths in three global pandemics. However, the functional role of lysine acetylation in this pathogen remains unclear. Here, we found more acetylated proteins and a higher degree of acetylation in Y. pestis grown under mammalian host (Mh) conditions than under flea vector (Fv) conditions, suggesting that protein acetylation could significantly change during fleabite transmission. Comparative acetylome analysis of mutants of YfiQ and CobB, the major acetyltransferase and deacetylase of Y. pestis, respectively, identified 23 YfiQ-dependent and 315 CobB-dependent acetylated proteins. Further results demonstrated that acetylation of Lys73 of the SlyA protein, a MarR-family transcriptional regulator, inhibits its binding to the promoter of target genes, including hmsT that encodes diguanylate cyclase responsible for the synthesis of c-di-GMP, and significantly enhances biofilm formation of Y. pestis. Our study presents the first extensive acetylome data of Y. pestis and a critical resource for the functional study of lysine acetylation in this pathogen. IMPORTANCE Yersinia pestis is the etiological agent of plague, historically responsible for three global pandemics. The 2017 plague epidemic in Madagascar was a reminder that Y. pestis remains a real threat in many parts of the world. Plague is a zoonotic disease that primarily infects rodents via fleabite, and transmission of Y. pestis from infected fleas to mammals requires rapid adaptive responses to adverse host environments to establish infection. Our study provides the first global profiling of lysine acetylation derived from mass spectrometry analysis in Y. pestis. Our data set can serve as a critical resource for the functional study of lysine acetylation in Y. pestis and provides new molecular insight into the physiological role of lysine acetylation in proteins. More importantly, we found that acetylation of Lys73 of SlyA significantly promotes biofilm formation of Y. pestis, indicating that bacteria can use lysine acetylation to fine-tune the expression of genes to improve adaptation.

Transcriptional Regulation of hmsB, A Temperature-Dependent Small RNA, by RovM in Yersinia pestis Biovar Microtus

HmsB, a temperature-dependent sRNA, promotes biofilm formation by Yersinia pestis, but whether its own expression is regulated by other regulators is still poorly understood. RovM is a global regulator that activates biofilm formation but represses the virulence of Y. pestis. In this work, the results of primer extension, quantitative real-time PCR (qRT-PCR), and LacZ fusion demonstrated that RovM was able to activate hmsB expression. However, the results of electrophoretic mobility shift assay (EMSA) showed that His-RovM did not bind to the upstream DNA region of hmsB. Thus, RovM may exert its regulatory action on hmsB expression in an indirect manner. The data presented here enriched the content of the regulatory circuits that control gene expression in Y. pestis.

Nlp enhances biofilm formation by Yersinia pestis biovar microtus

Biofilms formed by Yersinia pestis are able to attach to and block flea's proventriculus, which stimulates the transmission of this pathogen from fleas to mammals. In this study, we found that Nlp (YP1143) enhanced biofilm formation by Y. pestis and had regulatory effects on biofilm-associated genes at the transcriptional level. Phenotypic assays, including colony morphology assay, crystal violet staining, and Caenorhabditis elegans biofilm assay, disclosed that Nlp strongly promoted biofilm formation by Y. pestis. Further gene regulation assays showed that Nlp stimulated the expression of hmsHFRS, hmsCDE and hmsB, while had no regulatory effect on the expression of hmsT and hmsP at the transcriptional level. These findings promoted us to gain more understanding of the complex regulatory circuits controlling biofilm formation by Y. pestis.

Cpx-signalling facilitates Hms-dependent biofilm formation by Yersinia pseudotuberculosis

Bacteria often reside in sessile communities called biofilms, where they adhere to a variety of surfaces and exist as aggregates in a viscous polymeric matrix. Biofilms are resistant to antimicrobial treatments, and are a major contributor to the persistence and chronicity of many bacterial infections. Herein, we determined that the CpxA-CpxR two-component system influenced the ability of enteropathogenic Yersinia pseudotuberculosis to develop biofilms. Mutant bacteria that accumulated the active CpxR~P isoform failed to form biofilms on plastic or on the surface of the Caenorhabditis elegans nematode. A failure to form biofilms on the worm surface prompted their survival when grown on the lawns of Y. pseudotuberculosis. Exopolysaccharide production by the hms loci is the major driver of biofilms formed by Yersinia. We used a number of molecular genetic approaches to demonstrate that active CpxR~P binds directly to the promoter regulatory elements of the hms loci to activate the repressors of hms expression and to repress the activators of hms expression. Consequently, active Cpx-signalling culminated in a loss of exopolysaccharide production. Hence, the development of Y. pseudotuberculosis biofilms on multiple surfaces is controlled by the Cpx-signalling, and at least in part this occurs through repressive effects on the Hms-dependent exopolysaccharide production.

Molecular and Genetic Mechanisms That Mediate Transmission of Yersinia pestis by Fleas

The ability to cause plague in mammals represents only half of the life history of Yersinia pestis. It is also able to colonize and produce a transmissible infection in the digestive tract of the flea, its insect host. Parallel to studies of the molecular mechanisms by which Y. pestis is able to overcome the immune response of its mammalian hosts, disseminate, and produce septicemia, studies of Y. pestis-flea interactions have led to the identification and characterization of important factors that lead to transmission by flea bite. Y. pestis adapts to the unique conditions in the flea gut by altering its metabolic physiology in ways that promote biofilm development, a common strategy by which bacteria cope with a nutrient-limited environment. Biofilm localization to the flea foregut disrupts normal fluid dynamics of blood feeding, resulting in regurgitative transmission. Many of the important genes, regulatory pathways, and molecules required for this process have been identified and are reviewed here.

Altered Yersinia pestis virulence is associated with the small regulatory RNA HmsA encoded on the plasmid pPCP1

Aim: The aim of this study was to access the effect of HmsA, a 65-nt small regulatory RNA encoded by the pPCP1 plasmid, on Yersinia pestis virulence. Materials & methods: Survival and the competition index were determined in mice infected with wild-type Y. pestis and an hmsA deletion mutant. RNA-seq was used to identify HmsA-regulated genes. Results: HmsA deletion enhanced Y. pestis virulence. However, there was no overlap between 18 upregulated genes associated with pathogenicity and potential direct HmsA targets, based on gene expression screening after HmsA-pulse overexpression. Conclusion: HmsA inhibits Y. pestis virulence, but this effect may be mediated by indirect effects on pathogenesis, iron homeostasis and/or other cellular processes.

The Cyclic AMP Receptor Protein Regulates Quorum Sensing and Global Gene Expression in Yersinia pestis during Planktonic Growth and Growth in Biofilms

Cyclic AMP (cAMP) receptor protein (Crp) is an important transcriptional regulator of Yersinia pestis Expression of crp increases during pneumonic plague as the pathogen depletes glucose and forms large biofilms within lungs. To better understand control of Y. pestis Crp, we determined a 1.8-Å crystal structure of the protein-cAMP complex. We found that compared to Escherichia coli Crp, C helix amino acid substitutions in Y. pestis Crp did not impact the cAMP dependency of Crp to bind DNA promoters. To investigate Y. pestis Crp-regulated genes during plague pneumonia, we performed RNA sequencing on both wild-type and Δcrp mutant bacteria growing in planktonic and biofilm states in minimal media with glucose or glycerol. Y. pestis Crp was found to dramatically alter expression of hundreds of genes in a manner dependent upon carbon source and growth state. Gel shift assays confirmed direct regulation of the malT and ptsG promoters, and Crp was then linked to Y. pestis growth on maltose as a sole carbon source. Iron regulation genes ybtA and fyuA were found to be indirectly regulated by Crp. A new connection between carbon source and quorum sensing was revealed as Crp was found to regulate production of acyl-homoserine lactones (AHLs) through direct and indirect regulation of genes for AHL synthetases and receptors. AHLs were subsequently identified in the lungs of Y. pestis-infected mice when crp expression was highest in Y. pestis biofilms. Thus, in addition to the well-studied pla gene, other Crp-regulated genes likely have important functions during plague infection.IMPORTANCE Bacterial pathogens have evolved extensive signaling pathways to translate environmental signals into changes in gene expression. While Crp has long been appreciated for its role in regulating metabolism of carbon sources in many bacterial species, transcriptional profiling has revealed that this protein regulates many other aspects of bacterial physiology. The plague pathogen Y. pestis requires this global regulator to survive in blood, skin, and lungs. During disease progression, this organism adapts to changes within these niches. In addition to regulating genes for metabolism of nonglucose sugars, we found that Crp regulates genes for virulence, metal acquisition, and quorum sensing by direct or indirect mechanisms. Thus, this single transcriptional regulator, which responds to changes in available carbon sources, can regulate multiple critical behaviors for causing disease.

An Osmoregulatory Mechanism Operating through OmpR and LrhA Controls the Motile-Sessile Switch in the Plant Growth-Promoting Bacterium Pantoea alhagi

Adaptation to osmotic stress is crucial for bacterial growth and survival in changing environments. Although a large number of osmotic stress response genes have been identified in various bacterial species, how osmotic changes affect bacterial motility, biofilm formation, and colonization of host niches remains largely unknown. In this study, we report that the LrhA regulator is an osmoregulated transcription factor that directly binds to the promoters of the flhDC, eps, and opgGH operons and differentially regulates their expression, thus inhibiting motility and promoting exopolysaccharide (EPS) production, synthesis of osmoregulated periplasmic glucans (OPGs), biofilm formation, and root colonization of the plant growth-promoting bacterium Pantoea alhagi LTYR-11Z. Further, we observed that the LrhA-regulated OPGs control RcsCD-RcsB activation in a concentration-dependent manner, and a high concentration of OPGs induced by increased medium osmolarity is maintained to achieve the high level of activation of the Rcs phosphorelay, which results in enhanced EPS synthesis and decreased motility in P. alhagi Moreover, we showed that the osmosensing regulator OmpR directly binds to the promoter of lrhA and promotes its expression, while lrhA expression is feedback inhibited by the activated Rcs phosphorelay system. Overall, our data support a model whereby P. alhagi senses environmental osmolarity changes through the EnvZ-OmpR two-component system and LrhA to regulate the synthesis of OPGs, EPS production, and flagellum-dependent motility, thereby employing a hierarchical signaling cascade to control the transition between a motile lifestyle and a biofilm lifestyle.IMPORTANCE Many motile bacterial populations form surface-attached biofilms in response to specific environmental cues, including osmotic stress in a range of natural and host-related systems. However, cross talk between bacterial osmosensing, swimming, and biofilm formation regulatory networks is not fully understood. Here, we report that the pleiotropic regulator LrhA in Pantoea alhagi is involved in the regulation of flagellar motility, biofilm formation, and host colonization and responds to osmotic upshift. We further show that this sensing relies on the EnvZ-OmpR two-component system that was known to detect changes in external osmotic stress. The EnvZ-OmpR-LrhA osmosensing signal transduction cascade is proposed to increase bacterial fitness under hyperosmotic conditions inside the host. Our work proposes a novel regulatory mechanism that links osmosensing and motile-sessile lifestyle transitions, which may provide new approaches to prevent or promote the formation of biofilms and host colonization in P. alhagi and other bacteria possessing a similar osmoregulatory mechanism.

The Yersinia pseudotuberculosis Cpx envelope stress system contributes to transcriptional activation of rovM

The Gram-negative enteropathogen Yersinia pseudotuberculosis possesses a number of regulatory systems that detect cell envelope damage caused by noxious extracytoplasmic stresses. The CpxA sensor kinase and CpxR response regulator two-component regulatory system is one such pathway. Active Cpx signalling upregulates various factors designed to repair and restore cell envelope integrity. Concomitantly, this pathway also down-regulates key determinants of virulence. In Yersinia, cpxA deletion accumulates high levels of phosphorylated CpxR (CpxR~P). Accumulated CpxR~P directly repressed rovA expression and this limited expression of virulence-associated processes. A second transcriptional regulator, RovM, also negatively regulates rovA expression in response to nutrient stress. Hence, this study aimed to determine if CpxR~P can influence rovA expression through control of RovM levels. We determined that the active CpxR~P isoform bound to the promoter of rovM and directly induced its expression, which naturally associated with a concurrent reduction in rovA expression. Site-directed mutagenesis of the CpxR~P binding sequence in the rovM promoter region desensitised rovM expression to CpxR~P. These data suggest that accumulated CpxR~P inversely manipulates the levels of two global transcriptional regulators, RovA and RovM, and this would be expected to have considerable influence on Yersinia pathophysiology and metabolism.

Transcriptional regulation of Yersinia pestis biofilm formation

Yersinia pestis, the causative agent of plague, is transmitted primarily by infected fleas in nature. Y. pestis can produce biofilms that block flea's proventriculus and promote flea-borne transmission. Transcriptional regulation of Y. pestis biofilm formation plays an important role in the response to complex changes in environments, including temperature, pH, oxidative stress, and restrictive nutrition conditions, and contributes to Y. pestis growth, reproduction, transmission, and pathogenesis. A set of transcriptional regulators involved in Y. pestis biofilm production simultaneously controls a variety of biological functions and physiological pathways. Interactions between these regulators contribute to the development of Y. pestis gene regulatory networks, which are helpful for a quick response to complex environmental changes and better survival. The roles of crucial factors and regulators involved in response to complex environmental signals and Y. pestis biofilm formation as well as the precise gene regulatory networks are discussed in this review, which will give a better understanding of the complicated mechanisms of transcriptional regulation in Y. pestis biofilm formation.

Differential Gene Expression Patterns of Yersinia pestis and Yersinia pseudotuberculosis during Infection and Biofilm Formation in the Flea Digestive Tract

Yersinia pestis, the etiologic agent of plague, emerged as a fleaborne pathogen only within the last 6,000 years. Just five simple genetic changes in the Yersinia pseudotuberculosis progenitor, which served to eliminate toxicity to fleas and to enhance survival and biofilm formation in the flea digestive tract, were key to the transition to the arthropodborne transmission route. To gain a deeper understanding of the genetic basis for the development of a transmissible biofilm infection in the flea foregut, we evaluated additional gene differences and performed in vivo transcriptional profiling of Y. pestis, a Y. pseudotuberculosis wild-type strain (unable to form biofilm in the flea foregut), and a Y. pseudotuberculosis mutant strain (able to produce foregut-blocking biofilm in fleas) recovered from fleas 1 day and 14 days after an infectious blood meal. Surprisingly, the Y. pseudotuberculosis mutations that increased c-di-GMP levels and enabled biofilm development in the flea did not change the expression levels of the hms genes responsible for the synthesis and export of the extracellular polysaccharide matrix required for mature biofilm formation. The Y. pseudotuberculosis mutant uniquely expressed much higher levels of Yersinia type VI secretion system 4 (T6SS-4) in the flea, and this locus was required for flea blockage by Y. pseudotuberculosis but not for blockage by Y. pestis. Significant differences between the two species in expression of several metabolism genes, the Psa fimbrial genes, quorum sensing-related genes, transcription regulation genes, and stress response genes were evident during flea infection. IMPORTANCE Y. pestis emerged as a highly virulent, arthropod-transmitted pathogen on the basis of relatively few and discrete genetic changes from Y. pseudotuberculosis. Parallel comparisons of the in vitro and in vivo transcriptomes of Y. pestis and two Y. pseudotuberculosis variants that produce a nontransmissible infection and a transmissible infection of the flea vector, respectively, provided insights into how Y. pestis has adapted to life in its flea vector and point to evolutionary changes in the regulation of metabolic and biofilm development pathways in these two closely related species.

Transcriptional Regulation Between the Two Global Regulators RovA and CRP in Yersinia pestis biovar Microtus

Yersinia pestis is a dangerous bacterial pathogen that can cause plague. Both RovA and cyclic AMP receptor protein (cAMP-CRP) are required for regulating biofilm- and virulence-related genes in Y. pestis. In this study, the transcriptional regulation between RovA and cAMP-CRP were analyzed by using primer extension, quantitative RT-PCR, LacZ fusion, and electrophoretic mobility shift assay. The results indicated that RovA repressed crp transcription in an indirect manner, while that RovA had no regulatory action on cyaA at the transcriptional level. In addition, cAMP-CRP did not regulate the transcription of rovA. Taken together with our previous results, complex regulatory interactions of RovA, cAMP-CRP, and PhoP/PhoQ in Y. pestis were revealed, which would promote us gain deeper understanding about coordinative modulation of biofilm- and virulence-related regulator genes.

Protein Acetylation Mediated by YfiQ and CobB Is Involved in the Virulence and Stress Response of Yersinia pestis

Recent studies revealed that acetylation is a widely used protein modification in prokaryotic organisms. The major protein acetylation acetyltransferase YfiQ and the sirtuin-like deacetylase CobB have been found to be involved in basic physiological processes, such as primary metabolism, chemotaxis, and stress responses, in Escherichia coli and Salmonella However, little is known about protein acetylation modifications in Yersinia pestis, a lethal pathogen responsible for millions of human deaths in three worldwide pandemics. Here we found that Yp_0659 and Yp_1760 of Y. pestis encode the major protein acetylation acetyltransferase YfiQ and the sirtuin-like deacetylase CobB, respectively, which can acetylate and deacetylate PhoP enzymatically in vitro Protein acetylation impairment in cobB and yfiQ mutants greatly decreased bacterial tolerance to cold, hot, high-salt, and acidic environments. Our comparative transcriptomic data revealed that the strongly decreased tolerance to stress stimuli was probably related to downregulation of the genes encoding the heat shock proteins (HtpG, HslV, HslR, and IbpA), cold shock proteins (CspC and CspA1), and acid resistance proteins (HdeB and AdiA). We found that the reversible acetylation mediated by CobB and YfiQ conferred attenuation of virulence, probably partially due to the decreased expression of the psaABCDEF operon, which encodes Psa fimbriae that play a key role in virulence of Y. pestis This is the first report, to our knowledge, on the roles of protein acetylation modification in stress responses, biofilm formation, and virulence of Y. pestis.

"Fleaing" the Plague: Adaptations of Yersinia pestis to Its Insect Vector That Lead to Transmission

Interest in arthropod-borne pathogens focuses primarily on how they cause disease in humans. How they produce a transmissible infection in their arthropod host is just as critical to their life cycle, however. Yersinia pestis adopts a unique life stage in the digestive tract of its flea vector, characterized by rapid formation of a bacterial biofilm that is enveloped in a complex extracellular polymeric substance. Localization and adherence of the biofilm to the flea foregut is essential for transmission. Here, we review the molecular and genetic mechanisms of these processes and present a comparative evaluation and updated model of two related transmission mechanisms.

A starvation-induced regulator, RovM, acts as a switch for planktonic/biofilm state transition in Yersinia pseudotuberculosis

The transition between the planktonic state and the biofilm-associated state is a key developmental decision for pathogenic bacteria. Biofilm formation by Yersinia pestis is regulated by hmsHFRS genes (β-1, 6-N-acetyl-D-glucosamine synthesis operon) in its flea vector and in vitro. However, the mechanism of biofilm formation in Yersinia pseudotuberculosis remains elusive. In this study, we demonstrate that the LysR-type regulator RovM inversely regulates biofilm formation and motility in Y. pseudotuberculosis by acting as a transcriptional regulator of these two functions. RovM is strongly induced during growth in minimal media but strongly repressed in complex media. On one hand, RovM enhances bacterial motility by activating the expression of FlhDC, the master regulator of flagellar genes, via the recognition of an operator upstream of the flhDC promoter. On the other hand, RovM represses β-GlcNAc production under nutrition-limited conditions, negatively regulating hmsHFRS expression by directly binding to the -35 element of its promoter. Compared to wild-type bacteria, the rovM mutant established denser biofilms and caused more extensive mortality in mice and silkworm larvae. These results indicate that RovM acts as a molecular switch to coordinate the expression of genes involved in biofilm formation and motility in response to the availability of nutrients.

Activation of leuO by LrhA in Escherichia coli

LeuO is a conserved LysR-type transcription factor of pleiotropic function in Enterobacteria. Regulation of the leuO gene has been best studied in Escherichia coli and Salmonella enterica. Its expression is silenced by the nucleoid-associated proteins H-NS and StpA, autoregulated by LeuO, and in E. coli activated by the transcription regulator BglJ-RcsB. However, signals which induce leuO expression remain unknown. Here we show that LrhA, a conserved LysR-type transcription regulator, activates leuO in E. coli. LrhA specifically binds the leuO regulatory region and activates expression of leuO from three promoters. Activation of leuO by LrhA is synergistic with activation by BglJ-RcsB, while co-regulation by LrhA, LeuO and H-NS/StpA suggests a complex regulatory interplay. In addition, hyperactive LrhA mutants including LrhA-12DN, 221TA, 61HR/221TA and 303DG were identified. Regulation of leuO by LrhA reveals a connection between the two pleiotropic regulators LeuO and LrhA in E. coli.

Plasmid pPCP1-derived sRNA HmsA promotes biofilm formation of Yersinia pestis

Background

The ability of Yersinia pestis to form a biofilm is an important characteristic in flea transmission of this pathogen. Y. pestis laterally acquired two plasmids (pPCP1and pMT1) and the ability to form biofilms when it evolved from Yersinia pseudotuberculosis. Small regulatory RNAs (sRNAs) are thought to play a crucial role in the processes of biofilm formation and pathogenesis.

Results

A pPCP1-derived sRNA HmsA (also known as sR084) was found to contribute to the enhanced biofilm formation phenotype of Y. pestis. The concentration of c-di-GMP was significantly reduced upon deletion of the hmsA gene in Y. pestis. The abundance of mRNA transcripts determining exopolysaccharide production, crucial for biofilm formation, was measured by primer extension, RT-PCR and lacZ transcriptional fusion assays in the wild-type and hmsA mutant strains. HmsA positively regulated biofilm synthesis-associated genes (hmsHFRS, hmsT and hmsCDE), but had no regulatory effect on the biofilm degradation-associated gene hmsP. Interestingly, the recently identified biofilm activator sRNA, HmsB, was rapidly degraded in the hmsA deletion mutant. Two genes (rovM and rovA) functioning as biofilm regulators were also found to be regulated by HmsA, whose regulatory effects were consistent with the HmsA-mediated biofilm phenotype.

Conclusion

HmsA potentially functions as an activator of biofilm formation in Y. pestis, implying that sRNAs encoded on the laterally acquired plasmids might be involved in the chromosome-based regulatory networks implicated in Y. pestis-specific physiological processes.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-016-0793-5) contains supplementary material, which is available to authorized users.