monitoring-intraspecies-competition-bacterial-cell-population

Monitoring Intraspecies Competition in a Bacterial Cell Population by Cocultivation of Fluorescently Labelled Strains

Many microorganisms such as bacteria proliferate extremely fast and the populations may reach high cell densities. Small fractions of cells in a ...

Georg-August University

Glutamate is a central metabolite in all organisms since it provides the link between carbon and nitrogen metabolism. In Bacillus subtilis, glutamate is synthesized exclusively by the glutamate synthase, and it can be degraded by the glutamate dehydrogenase. In B. subtilis, the major glutamate dehydrogenase RocG is expressed only in the presence of arginine, and the bacteria are unable to utilize glutamate as the only carbon source. In addition to rocG, a second cryptic gene (gudB) encodes an inactive glutamate dehydrogenase. Mutations in rocG result in the rapid accumulation of gudB1 suppressor mutations that code for an active enzyme. In this work, we analyzed the physiological significance of this constellation of genes and enzymes involved in glutamate metabolism. We found that the weak expression of rocG in the absence of the inducer arginine is limiting for glutamate utilization. Moreover, we addressed the potential ability of the active glutamate dehydrogenases of B. subtilis to synthesize glutamate. Both RocG and GudB1 were unable to catalyze the anabolic reaction, most probably because of their very high K(m) values for ammonium. In contrast, the Escherichia coli glutamate dehydrogenase is able to produce glutamate even in the background of a B. subtilis cell. B. subtilis responds to any mutation that interferes with glutamate metabolism with the rapid accumulation of extragenic or intragenic suppressor mutations, bringing the glutamate supply into balance. Similarly, with the presence of a cryptic gene, the system can flexibly respond to changes in the external glutamate supply by the selection of mutations.

Glutamate metabolism in Bacillus subtilis: gene expression and enzyme activities evolved to avoid futile cycles and to allow rapid responses to perturbations of the system.

Any signal transduction requires communication between a sensory component and an effector. Some enzymes engage in signal perception and transduction, as well as in catalysis, and these proteins are known as "trigger" enzymes. In this report, we detail the trigger properties of RocG, the glutamate dehydrogenase of Bacillus subtilis. RocG not only deaminates the key metabolite glutamate to form alpha-ketoglutarate but also interacts directly with GltC, a LysR-type transcription factor that regulates glutamate biosynthesis from alpha-ketoglutarate, thus linking the two metabolic pathways. We have isolated mutants of RocG that separate the two functions. Several mutations resulted in permanent inactivation of GltC as long as a source of glutamate was present. These RocG proteins have lost their ability to catabolize glutamate due to a strongly reduced affinity for glutamate. The second class of mutants is exemplified by the replacement of aspartate residue 122 by asparagine. This mutant protein has retained enzymatic activity but has lost the ability to control the activity of GltC. Crystal structures of glutamate dehydrogenases that permit a molecular explanation of the properties of the various mutants are presented. Specifically, we may propose that D122N replacement affects the surface of RocG. Our data provide evidence for a correlation between the enzymatic activity of RocG and its ability to inactivate GltC, and thus give insights into the mechanism that couples the enzymatic activity of a trigger enzyme to its regulatory function.

Functional dissection of a trigger enzyme: mutations of the bacillus subtilis glutamate dehydrogenase RocG that affect differentially its catalytic activity and regulatory properties.

Several computational methods exist to suggest rational genetic interventions that improve the productivity of industrial strains. Nonetheless, these methods are less effective to predict possible genetic responses of the strain after the intervention. This problem requires a better understanding of potential alternative metabolic and regulatory pathways able to counteract the targeted intervention.

SPABBATS: A pathway-discovery method based on Boolean satisfiability that facilitates the characterization of suppressor mutants.

Cells of Bacillus subtilis can either be motile or sessile, depending on the expression of mutually exclusive sets of genes that are required for flagellum or biofilm formation, respectively. Both activities are coordinated by the master regulator SinR. We have analyzed the role of the previously uncharacterized ymdB gene for bistable gene expression in B. subtilis. We observed a strong overexpression of the hag gene encoding flagellin and of other genes of the σ(D)-dependent motility regulon in the ymdB mutant, whereas the two major operons for biofilm formation, tapA-sipW-tasA and epsA-O, were not expressed. As a result, the ymdB mutant is unable to form biofilms. An analysis of the individual cells of a population revealed that the ymdB mutant no longer exhibited bistable behavior; instead, all cells are short and motile. The inability of the ymdB mutant to form biofilms is suppressed by the deletion of the sinR gene encoding the master regulator of biofilm formation, indicating that SinR-dependent repression of biofilm genes cannot be relieved in a ymdB mutant. Our studies demonstrate that lack of expression of SlrR, an antagonist of SinR, is responsible for the observed phenotypes. Overexpression of SlrR suppresses the effects of a ymdB mutation.

A novel factor controlling bistability in Bacillus subtilis: the YmdB protein affects flagellin expression and biofilm formation.

Common laboratory strains of Bacillus subtilis encode two glutamate dehydrogenases: the enzymatically active protein RocG and the cryptic enzyme GudB that is inactive due to a duplication of three amino acids in its active center. The inactivation of the rocG gene results in poor growth of the bacteria on complex media due to the accumulation of toxic intermediates. Therefore, rocG mutants readily acquire suppressor mutations that decryptify the gudB gene. This decryptification occurs by a precise deletion of one part of the 9-bp direct repeat that causes the amino acid duplication. This mutation occurs at the extremely high frequency of 10(-4). Mutations affecting the integrity of the direct repeat result in a strong reduction of the mutation frequency; however, the actual sequence of the repeat is not essential. The mutation frequency of gudB was not affected by the position of the gene on the chromosome. When the direct repeat was placed in the completely different context of an artificial promoter, the precise deletion of one part of the repeat was also observed, but the mutation frequency was reduced by 3 orders of magnitude. Thus, transcription of the gudB gene seems to be essential for the high frequency of the appearance of the gudB1 mutation. This idea is supported by the finding that the transcription-repair coupling factor Mfd is required for the decryptification of gudB. The Mfd-mediated coupling of transcription to mutagenesis might be a built-in precaution that facilitates the accumulation of mutations preferentially in transcribed genes.

A high-frequency mutation in Bacillus subtilis: requirements for the decryptification of the gudB glutamate dehydrogenase gene.

Glutamate, the major amino group donor in anabolism, is synthesized by the combined action of the glutamine synthetase (GS) and the glutamate synthase (GOGAT) in Bacillus subtilis. The glutamate dehydrogenase (GDH) exclusively degrades glutamate. GS and GDH are both trigger enzymes, active in nitrogen metabolism and in controlling gene expression. Feedback-inhibited GS (FBI-GS) controls DNA-binding activities of two transcription factors, the repressor GlnR and TnrA, the global regulator of nitrogen metabolism. FBI-GS binds to and activates GlnR. This protein complex inhibits GS formation and thus glutamine synthesis. Moreover, FBI-GS inhibits DNA-binding activity of TnrA. Glutamate biosynthesis, the reaction linking carbon with nitrogen metabolism, is controlled by GDH. Together with glutamate GDH inhibits GltC, the transcription factor that activates expression of the GOGAT genes. Thus, GS and GDH control glutamine and glutamate synthesis, respectively, depending on the nitrogen status of the cell. B. subtilis lacking a functional GDH show a severe growth defect. Interestingly, the growth defect is suppressed by the rapid activation of an inactive GDH. Thus, maintenance of glutamate homeostasis is crucial for cellular vitality. This review covers the recent work on the complex control of glutamine and glutamate metabolism in the Gram-positive model organism B. subtilis.

Control of glutamate homeostasis in Bacillus subtilis: a complex interplay between ammonium assimilation, glutamate biosynthesis and degradation.

The genome of the Gram-positive soil bacterium Bacillus subtilis encodes three potential diadenylate cyclases that may synthesize the signaling nucleotide cyclic di-AMP (c-di-AMP). These enzymes are expressed under different conditions in different cell compartments, and they localize to distinct positions in the cell. Here we demonstrate the diadenylate cyclase activity of the so far uncharacterized enzymes CdaA (previously known as YbbP) and CdaS (YojJ). Our work confirms that c-di-AMP is essential for the growth of B. subtilis and shows that an excess of the molecule is also harmful for the bacteria. Several lines of evidence suggest that the diadenylate cyclase CdaA is part of the conserved essential cda-glm module involved in cell wall metabolism. In contrast, the CdaS enzyme seems to provide c-di-AMP for spores. Accumulation of large amounts of c-di-AMP impairs the growth of B. subtilis and results in the formation of aberrant curly cells. This phenotype can be partially suppressed by elevated concentrations of magnesium. These observations suggest that c-di-AMP interferes with the peptidoglycan synthesis machinery. The activity of the diadenylate cyclases is controlled by distinct molecular mechanisms. CdaA is stimulated by a regulatory interaction with the CdaR (YbbR) protein. In contrast, the activity of CdaS seems to be intrinsically restricted, and a single amino acid substitution is sufficient to drastically increase the activity of the enzyme. Taken together, our results support the idea of an important role for c-di-AMP in B. subtilis and suggest that the levels of the nucleotide have to be tightly controlled.

Cyclic di-AMP homeostasis in bacillus subtilis: both lack and high level accumulation of the nucleotide are detrimental for cell growth.

Soil bacteria like Bacillus subtilis can cope with many growth conditions by adjusting gene expression and metabolic pathways. Alternatively, bacteria can spontaneously accumulate beneficial mutations or shape their genomes in response to stress. Recently, it has been observed that a B. subtilis mutant lacking the catabolically active glutamate dehydrogenase (GDH), RocG, mutates the cryptic gudB(CR) gene at a high frequency. The suppressor mutants express the active GDH GudB, which can fully replace the function of RocG. Interestingly, the cryptic gudB(CR) allele is stably inherited as long as the bacteria synthesize the functional GDH RocG. Competition experiments revealed that the presence of the cryptic gudB(CR) allele provides the bacteria with a selective growth advantage when glutamate is scarce. Moreover, the lack of exogenous glutamate is the driving force for the selection of mutants that have inactivated the active gudB gene. In contrast, two functional GDHs are beneficial for the cells when glutamate was available. Thus, the amount of GDH activity strongly affects fitness of the bacteria depending on the availability of exogenous glutamate. At a first glance the high mutation frequency of the cryptic gudB(CR) allele might be attributed to stress-induced adaptive mutagenesis. However, other loci on the chromosome that could be potentially mutated during growth under the selective pressure that is exerted on a GDH-deficient mutant remained unaffected. Moreover, we show that a GDH-proficient B. subtilis strain has a strong selective growth advantage in a glutamate-dependent manner. Thus, the emergence and rapid clonal expansion of the active gudB allele can be in fact explained by spontaneous mutation and growth under selection without an increase of the mutation rate. Moreover, this study shows that the selective pressure that is exerted on a maladapted bacterium strongly affects the apparent mutation frequency of mutational hot spots.

Selection-driven accumulation of suppressor mutants in bacillus subtilis: the apparent high mutation frequency of the cryptic gudB gene and the rapid clonal expansion of gudB(+) suppressors are due to growth under selection.

PutP and OpuE serve as proline transporters when this imino acid is used by Bacillus subtilis as a nutrient or as an osmostress protectant, respectively. The simultaneous inactivation of the PutP and OpuE systems still allows the utilization of proline as a nutrient. This growth phenotype pointed to the presence of a third proline transport system in B. subtilis. We took advantage of the sensitivity of a putP opuE double mutant to the toxic proline analog 3,4-dehydro-dl-proline (DHP) to identify this additional proline uptake system. DHP-resistant mutants were selected and found to be defective in the use of proline as a nutrient. Whole-genome resequencing of one of these strains provided the lead that the inactivation of the γ-aminobutyrate (GABA) transporter GabP was responsible for these phenotypes. DNA sequencing of the gabP gene in 14 additionally analyzed DHP-resistant strains confirmed this finding. Consistently, each of the DHP-resistant mutants was defective not only in the use of proline as a nutrient but also in the use of GABA as a nitrogen source. The same phenotype resulted from the targeted deletion of the gabP gene in a putP opuE mutant strain. Hence, the GabP carrier not only serves as an uptake system for GABA but also functions as the third proline transporter of B. subtilis. Uptake studies with radiolabeled GABA and proline confirmed this conclusion and provided information on the kinetic parameters of the GabP carrier for both of these substrates.

Katrin Gunka

Department of General Microbiology

Georg-August University

Monitoring Intraspecies Competition in a Bacterial Cell Population by Cocultivation of Fluorescently Labelled Strains