Ca2+-dependent expression of the CIRCE regulon in Streptococcus pneumoniae

Diverse regulatory pathways modulate bet hedging of competence induction in epigenetically-differentiated phase variants of Streptococcus pneumoniae

Despite enabling Streptococcus pneumoniae to acquire antibiotic resistance and evade vaccine-induced immunity, transformation occurs at variable rates across pneumococci. Phase variants of isolate RMV7, distinguished by altered methylation patterns driven by the translocating variable restriction-modification (tvr) locus, differed significantly in their transformation efficiencies and biofilm thicknesses. These differences were replicated when the corresponding tvr alleles were introduced into an RMV7 derivative lacking the locus. RNA-seq identified differential expression of the type 1 pilus, causing the variation in biofilm formation, and inhibition of competence induction in the less transformable variant, RMV7domi. This was partly attributable to RMV7domi's lower expression of ManLMN, which promoted competence induction through importing N-acetylglucosamine. This effect was potentiated by analogues of some proteobacterial competence regulatory machinery. Additionally, one of RMV7domi's phage-related chromosomal island was relatively active, which inhibited transformation by increasing expression of the stress response proteins ClpP and HrcA. However, HrcA increased competence induction in the other variant, with its effects depending on Ca2+ supplementation and heat shock. Hence the heterogeneity in transformation efficiency likely reflects the diverse signalling pathways by which it is affected. This regulatory complexity will modulate population-wide responses to synchronising quorum sensing signals to produce co-ordinated yet stochastic bet hedging behaviour.

Mechanism of HrcA function in heat shock regulation in Mycobacterium tuberculosis

Molecular chaperones are a conserved family of proteins that are over-expressed in response to heat and other stresses. The regulation of expression of chaperone proteins plays a vital role in pathogenesis of various bacterial pathogens. In M. tuberculosis, HrcA and HspR negatively regulate heat shock protein operons by binding to their cognate DNA elements, CIRCE and HAIR respectively. In this study, we show that M. tuberculosis HrcA is able to bind to its cognate CIRCE DNA element present in the upstream regions of groES and groEL2 operons only with the help of other protein(s). It is also demonstrated that M. tuberculosis HrcA binds to a CIRCE like DNA element present in the upstream region of hrcA gene suggesting its auto-regulatory nature. In addition, we report the presence of a putative HAIR element in the upstream region of groES operon and demonstrate the binding of HspR to it. In vitro, HrcA inhibited the DNA binding activity of HspR in a dose-dependent manner. The current study demonstrates that M. tuberculosis HrcA requires other protein(s) to function, and the heat shock protein expression in M. tuberculosis is negatively regulated jointly by HrcA and HspR.

Co(2+)-dependent gene expression in Streptococcus pneumoniae: opposite effect of Mn(2+) and Co(2+) on the expression of the virulence genes psaBCA, pcpA, and prtA

Manganese (Mn(2+))-, zinc (Zn(2+))- and copper (Cu(2+)) play significant roles in transcriptional gene regulation, physiology, and virulence of Streptococcus pneumoniae. So far, the effect of the important transition metal ion cobalt (Co(2+)) on gene expression of S. pneumoniae has not yet been explored. Here, we study the impact of Co(2+) stress on the transcriptome of S. pneumoniae strain D39. BLAST searches revealed that the genome of S. pneumoniae encodes a putative Co(2+)-transport operon (cbi operon), the expression of which we show here to be induced by a high Co(2+) concentration. Furthermore, we found that Co(2+), as has been shown previously for Zn(2+), can cause derepression of the genes of the PsaR virulence regulon, encoding the Mn(2+)-uptake system PsaBCA, the choline binding protein PcpA and the cell-wall associated serine protease PrtA. Interestingly, although Mn(2+) represses expression of the PsaR regulon and Co(2+) leads to derepression, both metal ions stimulate interaction of PsaR with its target promoters. These data will be discussed in the light of previous studies on similar metal-responsive transcriptional regulators.

Calcium binding proteins and calcium signaling in prokaryotes

With the continued increase of genomic information and computational analyses during the recent years, the number of newly discovered calcium binding proteins (CaBPs) in prokaryotic organisms has increased dramatically. These proteins contain sequences that closely resemble a variety of eukaryotic calcium (Ca(2+)) binding motifs including the canonical and pseudo EF-hand motifs, Ca(2+)-binding β-roll, Greek key motif and a novel putative Ca(2+)-binding domain, called the Big domain. Prokaryotic CaBPs have been implicated in diverse cellular activities such as division, development, motility, homeostasis, stress response, secretion, transport, signaling and host-pathogen interactions. However, the majority of these proteins are hypothetical, and only few of them have been studied functionally. The finding of many diverse CaBPs in prokaryotic genomes opens an exciting area of research to explore and define the role of Ca(2+) in organisms other than eukaryotes. This review presents the most recent developments in the field of CaBPs and novel advancements in the role of Ca(2+) in prokaryotes.

Use of proteomic analysis to elucidate the role of calcium in acetone-butanol-ethanol fermentation by Clostridium beijerinckii NCIMB 8052

Calcium carbonate increases growth, substrate utilization, and acetone-butanol-ethanol (ABE) fermentation by Clostridium beijerinckii NCIMB 8052. Toward an understanding of the basis for these pleiotropic effects, we profiled changes in the C. beijerinckii NCIMB 8052 proteome that occur in response to the addition of CaCO(3). We observed increases in the levels of different heat shock proteins (GrpE and DnaK), sugar transporters, and proteins involved in DNA synthesis, repair, recombination, and replication. We also noted significant decreases in the levels of proteins involved in metabolism, nucleic acid stabilization, sporulation, oxidative and antibiotic stress responses, and signal transduction. We determined that CaCO(3) enhances ABE fermentation due to both its buffering effects and its ability to influence key cellular processes, such as sugar transport, butanol tolerance, and solventogenesis. Moreover, activity assays in vitro for select solventogenic enzymes revealed that part of the underpinning for the CaCO(3)-mediated increase in the level of ABE fermentation stems from the enhanced activity of these catalysts in the presence of Ca(2+). Collectively, these proteomic and biochemical studies provide new insights into the multifactorial basis for the stimulation of ABE fermentation and butanol tolerance in the presence of CaCO(3).

Reduction-sensitive and cysteine residue-mediated Streptococcus pneumoniae HrcA oligomerization in vitro

In both gram-positive and several gram-negative bacteria, the transcription of dnaK and groE operons is negatively regulated by HrcA; however, the mechanism modulating HrcA protein activity upon thermal stress remains elusive. Here, we demonstrate that HrcA is modulated via reduction and oligomerization in vitro. Native-PAGE analysis was used to reveal the oligomeric structure of HrcA. The oligomeric HrcA structure became monomeric following treatment with the reducing agent dithothreitol, and this process was reversed by treatment with hydrogen peroxide. Moreover, the mutant HrcA C118S exhibited reduced binding to CIRCE elements and became less oligomerized, suggesting that cysteine residue 118 is important for CIRCE element binding as well as oligomerization. Conversely, HrcA mutant C280S exhibited increased oligomerization. An HrcA double mutant (C118S, C280S) was monomeric and exhibited a level of oligomerization and CIRCE binding similar to wild type HrcA, suggesting that cysteine residues 118 and 280 may function as checks to one another during oligomer formation. Biochemical fractionation of E. coli cells overexpressing HrcA revealed the presence of HrcA in the membrane fraction. Together, these results suggest that the two HrcA cysteine residues at positions 118 and 280 function as reduction sensors in the membrane and mediate oligomerization upon stress.

Calcium efflux is essential for bacterial survival in the eukaryotic host

In dynamic environments, intracellular homeostasis is maintained by transport systems found in all cells. While bacterial influx systems for essential trace cations are known to contribute to pathogenesis, efflux systems have been characterized mainly in contaminated environmental sites. We describe that the high calcium concentrations in the normal human host were toxic to pneumococci and that bacterial survival in vivo depended on CaxP, the first Ca2+ exporter reported in bacteria. CaxP homologues were found in the eukaryotic sacroplasmic reticulum and in many bacterial genomes. A caxP- mutant accumulated intracellular calcium, a state that was used to reveal signalling networks responsive to changes in intracellular calcium concentration. Chemical inhibition of CaxP was bacteriostatic in physiological calcium concentrations, suggesting a new antibiotic target uncovered under conditions in the eukaryotic host.

Dual regulation of dnaK and groE operons by HrcA and Ca++ in Streptococcus pneumoniae

The dnaK and groE operons in Streptococcus pneumoniae are repressed by HrcA in the presence of Ca(++). However, it is unclear how HrcA and Ca(++) regulate the dnaK and groE operons. This study examined the dual regulation of the dnaK and groE operons in S. pneumoniae by HrcA and Ca(++). At 30 degrees C, the hrcA mutant showed a constitutively higher level of dnaK expression at both the protein and mRNA levels than that of the wild type whereas the level of groEL expression was relatively unchanged. On the other hand, the levels of both dnaK and groEL transcripts were increased after heat shock but the hrcA mutant was not sensitive to heat shock. Immunoblot analysis of the cells pretreated with the Ca(++) chelator, BAPTA-AM, revealed the induction of HrcA and GroEL at both 30 degrees C and 42 degrees C. However, at longer preincubation time with BAPTA-AM, GroEL was further induced but the level of HrcA decreased suggesting that Ca(++) regulates the dnaK and groE operons differently. Overall, Ca(++) and HrcA differentially regulate the transcription of the dnaK and groE operons in S. pneumoniae. These results are expected to contribute to resolving the relationship between DnaK/GroEL expression and the pathogenesis in S. pneumoniae.

The Xanthomonas oryzae pv. oryzae PhoPQ two-component system is required for AvrXA21 activity, hrpG expression, and virulence

The rice pathogen recognition receptor, XA21, confers resistance to Xanthomonas oryzae pv. oryzae strains producing the type one system-secreted molecule, AvrXA21. X. oryzae pv. oryzae requires a regulatory two-component system (TCS) called RaxRH to regulate expression of eight rax (required for AvrXA21 activity) genes and to sense population cell density. To identify other key components in this critical regulatory circuit, we assayed proteins expressed in a raxR gene knockout strain. This survey led to the identification of the phoP gene encoding a response regulator that is up-regulated in the raxR knockout strain. Next we generated a phoP knockout strain and found it to be impaired in X. oryzae pv. oryzae virulence and no longer able to activate the response regulator HrpG (hypersensitive reaction and pathogenicity G) in response to low levels of Ca2+. The impaired virulence of the phoP knockout strain can be partially complemented by constitutive expression of hrpG, indicating that PhoP controls a key aspect of X. oryzae pv. oryzae virulence through regulation of hrpG. A gene encoding the cognate putative histidine protein kinase, phoQ, was also isolated. Growth curve analysis revealed that AvrXA21 activity is impaired in a phoQ knockout strain as reflected by enhanced growth of this strain in rice lines carrying XA21. These results suggest that the X. oryzae pv. oryzae PhoPQ TCS functions in virulence and in the production of AvrXA21 in partnership with RaxRH.