Expression of genes coding for GerA and GerK spore germination receptors is dependent on the protein phosphatase PrpE

Spore Germination

Despite being resistant to a variety of environmental insults, the bacterial endospore can sense the presence of small molecules and respond by germinating, losing the specialized structures of the dormant spore, and resuming active metabolism, before outgrowing into vegetative cells. Our current level of understanding of the spore germination process in bacilli and clostridia is reviewed, with particular emphasis on the germinant receptors characterized in Bacillus subtilis, Bacillus cereus, and Bacillus anthracis. The recent evidence for a local clustering of receptors in a "germinosome" would begin to explain how signals from different receptors could be integrated. The SpoVA proteins, involved in the uptake of Ca2+-dipicolinic acid into the forespore during sporulation, are also responsible for its release during germination. Lytic enzymes SleB and CwlJ, found in bacilli and some clostridia, hydrolyze the spore cortex: other clostridia use SleC for this purpose. With genome sequencing has come the appreciation that there is considerable diversity in the setting for the germination machinery between bacilli and clostridia.

Role of Alanine Dehydrogenase of Mycobacterium tuberculosis during Recovery from Hypoxic Nonreplicating Persistence

Mycobacterium tuberculosis can maintain a nonreplicating persistent state in the host for decades, but must maintain the ability to efficiently reactivate and produce active disease to survive and spread in a population. Among the enzymes expressed during this dormancy is alanine dehydrogenase, which converts pyruvate to alanine, and glyoxylate to glycine concurrent with the oxidation of NADH to NAD. It is involved in the metabolic remodeling of M. tuberculosis through its possible interactions with both the glyoxylate and methylcitrate cycle. Both mRNA levels and enzymatic activities of isocitrate lyase, the first enzyme of the glyoxylate cycle, and alanine dehydrogenase increased during entry into nonreplicating persistence, while the gene and activity for the second enzyme of the glyoxylate cycle, malate synthase were not. This could suggest a shift in carbon flow away from the glyoxylate cycle and instead through alanine dehydrogenase. Expression of ald was also induced in vitro by other persistence-inducing stresses such as nitric oxide, and was expressed at high levels in vivo during the initial lung infection in mice. Enzyme activity was maintained during extended hypoxia even after transcription levels decreased. An ald knockout mutant of M. tuberculosis showed no reduction in anaerobic survival in vitro, but resulted in a significant lag in the resumption of growth after reoxygenation. During reactivation the ald mutant had an altered NADH/NAD ratio, and alanine dehydrogenase is proposed to maintain the optimal NADH/NAD ratio during anaerobiosis in preparation of eventual regrowth, and during the initial response during reoxygenation.

A genome-wide transcriptional profiling of sporulating Bacillus subtilis strain lacking PrpE protein phosphatase

The sporulation process is a complex genetic developmental program leading to profound changes in global gene expression profile. In this work, we have applied genome-wide microarray approach for transcriptional profiling of Bacillus subtilis strain lacking a gene coding for PrpE protein phosphatase. This protein was previously shown to be involved in the regulation of germination of B. subtilis spores. Moreover, the deletion of prpE gene resulted in changing the resistance properties of spores. Our results provide genome-wide insight into the influence of this protein phosphatase on the physiology of B. subtilis cells. Although the precise role of PrpE in shaping the observed phenotype of ΔprpE mutant strain still remains beyond the understanding, our experiments brought observations of possible indirect implication of this protein in the regulation of cell motility and chemotaxis, as well as the development of competence. Surprisingly, prpE-deleted cells showed elevated level of general stress response, which turned out to be growth medium specific.

Live cell imaging of germination and outgrowth of individual bacillus subtilis spores; the effect of heat stress quantitatively analyzed with SporeTracker

Spore-forming bacteria are a special problem for the food industry as some of them are able to survive preservation processes. Bacillus spp. spores can remain in a dormant, stress resistant state for a long period of time. Vegetative cells are formed by germination of spores followed by a more extended outgrowth phase. Spore germination and outgrowth progression are often very heterogeneous and therefore, predictions of microbial stability of food products are exceedingly difficult. Mechanistic details of the cause of this heterogeneity are necessary. In order to examine spore heterogeneity we made a novel closed air-containing chamber for live imaging. This chamber was used to analyze Bacillus subtilis spore germination, outgrowth, as well as subsequent vegetative growth. Typically, we examined around 90 starting spores/cells for ≥4 hours per experiment. Image analysis with the purposely built program “SporeTracker” allows for automated data processing from germination to outgrowth and vegetative doubling. In order to check the efficiency of the chamber, growth and division of B. subtilis vegetative cells were monitored. The observed generation times of vegetative cells were comparable to those obtained in well-aerated shake flask cultures. The influence of a heat stress of 85°C for 10 min on germination, outgrowth, and subsequent vegetative growth was investigated in detail. Compared to control samples fewer spores germinated (41.1% less) and fewer grew out (48.4% less) after the treatment. The heat treatment had a significant influence on the average time to the start of germination (increased) and the distribution and average of the duration of germination itself (increased). However, the distribution and the mean outgrowth time and the generation time of vegetative cells, emerging from untreated and thermally injured spores, were similar.

Effects of the SpoVT regulatory protein on the germination and germination protein levels of spores of Bacillus subtilis

Bacillus subtilis isolates lacking the SpoVT protein, which regulates gene expression in developing forespores, gave spores that released their dipicolinic acid (DPA) via germinant receptor (GR)-dependent germination more rapidly than wild-type spores. Non-GR-dependent germination via dodecylamine was more rapid with spoVT spores, but germination via Ca-DPA was slower. The effects of a spoVT mutation on spore germination were seen with spores made in rich and poor media, and levels of SpoVT-LacZ were elevated 2-fold in poor-medium spores; however, elevated SpoVT levels were not the only cause of the slower GR-dependent germination of poor-medium spores. The spoVT spores had ≥5-fold higher GerA GR levels, ∼2-fold elevated GerB GR levels, wild-type levels of a GerK GR subunit and the GerD protein required for normal GR-dependent germination, ∼2.5-fold lower levels of the SpoVAD protein involved in DPA release in spore germination, and 30% lower levels of DNA protective α/β-type small, acid-soluble spore proteins. With one exception, the effects on protein levels in spoVT spores are consistent with the effects of SpoVT on forespore transcription. The spoVT spores were also more sensitive to UV radiation and outgrew slowly. While spoVT spores' elevated GR levels were consistent with their more rapid GR-dependent germination, detailed analysis of the results suggested that there is another gene product crucial for GR-dependent spore germination that is upregulated in the absence of SpoVT. Overall, these results indicate that SpoVT levels during spore formation have a major impact on the germination and the resistance of the resultant spores.

Germination protein levels and rates of germination of spores of Bacillus subtilis with overexpressed or deleted genes encoding germination proteins

Deletion of Bacillus subtilis spores' GerA germinant receptor (GR) had no effect on spore germination via the GerB plus GerK GRs, and loss of GerB plus GerK did not affect germination via GerA. Loss of one or two GRs also did not affect levels of GRs that were not deleted. Overexpression of GRs 5- to 18-fold increased rates of germination via the overexpressed GR and slowed germination by other GRs up to 15-fold. However, overexpression of one or two GRs had no effect on levels of GRs that were not overexpressed. These results suggest that either interaction between different GRs reduces the activity of GRs in triggering spore germination or all GRs compete for interaction with a limiting amount of a downstream signaling molecule in the germination pathway. Overexpression or deletion of GRs also had no effect on spores' levels of the GerD protein needed for normal GR-dependent germination or of the SpoVAD protein likely involved in dipicolinic acid release early in germination. Loss of GerD also had no effect on levels of GRs or SpoVAD. Spores of a strain lacking the only B. subtilis prelipoprotein diacylglycerol transferase, GerF, also had no detectable GerD or the GerA's C subunit, both of which are most likely lipoproteins; GerA's A subunit was also absent. However, levels of GerB's C subunit, also almost certainly a lipoprotein, and GerK's A subunit were normal in gerF spores. These results with gerF spores were consistent with effects of loss of GerF on spore germination by different GRs.

Effects of sporulation conditions on the germination and germination protein levels of Bacillus subtilis spores

Bacillus subtilis spores prepared in rich medium germinated faster with nutrient germinants than poor-medium spores as populations in liquid and multiple individual spores on a microscope slide. Poor-medium spores had longer average lag times between mixing of spores with nutrient germinants and initiation of Ca-dipicolinic acid (CaDPA) release. Rich-medium spores made at 37°C germinated slightly faster with nutrient germinants than 23°C spores in liquid, but not when spores germinated on a slide. The difference in germination characteristics of these spore populations in liquid was paralleled by changes in expression levels of a transcriptional lacZ fusion to the gerA operon, encoding a germinant receptor (GR). Levels of GR subunits were 3- to 8-fold lower in poor-medium spores than rich-medium spores and 1.6- to 2-fold lower in 23°C spores than 37°C spores, and levels of the auxiliary germination protein GerD were 3.5- to 4-fold lower in poor medium and 23°C spores. In contrast, levels of another likely germination protein, SpoVAD, were similar in all these spores. These different spores germinated similarly with CaDPA, and poor-medium and 23°C spores germinated faster than rich-medium and 37°C spores, respectively, with dodecylamine. Since spore germination with CaDPA and dodecylamine does not require GerD or GRs, these results indicate that determinants of rates of nutrient germination of spores prepared differently are primarily the levels of the GRs that bind nutrient germinants and trigger germination and secondarily the levels of GerD.

Effect of amino acid substitutions in the GerAA protein on the function of the alanine-responsive germinant receptor of Bacillus subtilis spores

Spores of Bacillus subtilis require the GerAA, GerAB, and GerAC receptor proteins for L-alanine-induced germination. Mutations in gerAA, both random and site directed, result in phenotypes that identify amino acid residues important for receptor function in broad terms. They highlight the functional importance of two regions in the central, integral membrane domain of GerAA. A P324S substitution in the first residue of a conserved PFPP motif results in a 10-fold increase in a spore's sensitivity to alanine; a P326S change results in the release of phase-dark spores, in which the receptor may be in an "activated" or "quasigerminated" state. Substitutions in residues 398 to 400, in a short loop between the last two likely membrane-spanning helices of this central domain, all affect the germination response, with the G398S substitution causing a temperature-sensitive defect. In others, there are wider effects on the receptor: if alanine is substituted for conserved residue N146, H304, or E330, a severe defect in L-alanine germination results. This correlates with the absence of GerAC, suggesting that the assembly or stability of the entire receptor complex has been compromised by the defect in GerAA. In contrast, severely germination-defective mutants such as E129K, L373F, S400F, and M409N mutants retain GerAC at normal levels, suggesting more local and specific effects on the function of GerAA itself. Further interpretation will depend on progress in structural analysis of the receptor proteins.

Proteomic analysis of small acid soluble proteins in the spore core of Bacillus subtilis ΔprpE and 168 strains with predictions of peptides liquid chromatography retention times as an additional tool in protein identification

Background

Sporulation, characteristic for some bacteria such as Bacillus subtilis, has not been entirely defined yet. Protein phosphatase E (PrpE) and small, acid soluble spore proteins (SASPs) influence this process. Nevertheless, direct result of PrpE interaction on SASPs content in spore coat of B. subtilis has not been evidenced so far. As proteomic approach enables global analysis of occurring proteins, therefore it was chosen in this experiment to compare SASPs occurrence in two strains of B. subtilis, standard 168 and ΔprpE, lacking PrpE phosphatase. Proteomic analysis is still a challenge, and despite of big approach in mass spectrometry (MS) field, the identification reliability remains unsatisfactory. Therefore there is a rising interest in new methods, particularly bioinformatic tools that would harden protein identification. Most of currently applied algorithms are based on MS-data. Information from separation steps is not still in routine usage, even though they also provide valuable facts about analyzed structures. The aim of this research was to apply a model for peptides retention times prediction, based on quantitative structure-retention relationships (QSRR) in SASPs analysis, obtained from two strains of B. subtilis proteome digests after separation and identification of the peptides by LC-ESI-MS/MS. The QSRR approach was applied as the additional constraint in proteomic research verifying results of MS/MS ion search and confirming the correctness of the peptides identifications along with the indication of the potential false positives and false negatives.

Results

In both strains of B. subtilis, peptides characteristic for SASPs were found, however their identification confidence varied. According to the MS identity parameter X_corr and difference between predicted and experimental retention times (Δt_R) four groups could be distinguished: correctly and incorrectly identified, potential false positives and false negatives. The ΔprpE strain was characterized by much higher amount of SASPs peptides than standard 168 and their identification confidence was, mostly for alpha- and beta-type SASP, satisfactory.

Conclusions

The QSRR-based model for predicting retention times of the peptides, was a useful additional to MS tool, enhancing protein identification. Higher content of SASPs in strain lacking PrpE phosphatase suggests that this enzyme may influence their occurrence in the spores, lowering levels of these proteins.

Phosphorylation and ATP-binding induced conformational changes in the PrkC, Ser/Thr kinase from B. subtilis

Recent studies on the PrkC, serine-threonine kinase show that that the enzyme is located at the inner membrane of endospores and is responsible for triggering spore germination. The activity of the protein increases considerably after phosphorylation of four threonine residues placed on the activation loop and one serine placed in the C-terminal lobe of the PrkC. The molecular relationship between phosphorylation of these residues and enzyme activity is not known. In this work molecular dynamics simulation is performed on four forms of the protein kinase PrkC from B. subtilis-phosphorylated or unphosphorylated; with or without ATP bound-in order to gain insight into phosphorylation and ATP binding on the conformational changes and functions of the protein kinase. Our results show how phosphorylation, as well as the presence of ATP, is important for the activity of the enzyme through its molecular interaction with the catalytic core residues. Three of four threonine residues were found to be involved in the interactions with conservative motifs important for the enzyme activity. Two of the threonine residues (T167 and T165) are involved in ionic interactions with an arginine cluster from alphaC-helix. The third residue (T163) plays a crucial role, interacting with His-Arg-Asp triad (HRD). Last of the threonine residues (T162), as well as the serine (S214), were indicated to play a role in the substrate recognition or dimerization of the enzyme. The presence of ATP in the unphosphorylated model induced conformational instability of the activation loop and Asp-Phe-Gly motif (DFG). Based on our calculations we put forward a hypothesis suggesting that the ATP binds after phosphorylation of the activation loop to create a fully active conformation in the closed position.

Identification of a glycosylated Ehrlichia canis 19-kilodalton major immunoreactive protein with a species-specific serine-rich glycopeptide epitope

Ehrlichia canis has a small subset of major immunoreactive proteins that includes a 19-kDa protein that elicits an early Ehrlichia-specific antibody response in infected dogs. We report herein the identification and molecular characterization of this highly conserved 19-kDa major immunoreactive glycoprotein (gp19) ortholog of the Ehrlichia chaffeensis variable-length PCR target (VLPT) protein. E. canis gp19 has substantial carboxyl-terminal amino acid homology (59%) with E. chaffeensis VLPT and the same chromosomal location; however, the E. chaffeensis VLPT gene (594 bp) has tandem repeats that are not present in the E. canis gp19 gene (414 bp). Consistent with other ehrlichial glycoproteins, the gp19 protein exhibited a larger-than-predicted mass (approximately 3 kDa), O-linked glycosylation sites were predicted in an amino-terminal serine/threonine/glutamate (STE)-rich patch (26 amino acids), carbohydrate was detected on the recombinant gp19 protein, and the neutral sugars glucose and galactose were detected on the recombinant amino-terminal polypeptide. E. canis gp19 composition consists of five predominant amino acids, cysteine, glutamate, tyrosine, serine, and threonine, concentrated in the STE-rich patch and a carboxyl-terminal domain predominated by cysteine and tyrosine (55%). The amino-terminal STE-rich patch contained a major species-specific antibody epitope strongly recognized by serum from an E. canis-infected dog. The recombinant glycopeptide epitope was substantially more reactive with antibody than the synthetic (nonglycosylated) peptide, and periodate treatment of the recombinant glycopeptide epitope reduced its immunoreactivity, demonstrating the importance of a carbohydrate immunodeterminant(s). The gp19 protein was present on reticulate and dense-cored cells, and it was found extracellularly in the fibrillar matrix and associated with the morula membrane, the host cell cytoplasm, and the nucleus.