A new selective force driving metabolic gene clustering.

The evolution of operons has puzzled evolutionary biologists since their discovery, and many theories exist to explain their emergence, spreading, and evolutionary conservation. In this work, we suggest that DNA replication introduces a selective force for the clustering of functionally related genes on chromosomes, which we interpret as a preliminary and necessary step in operon formation. Our reasoning starts from the observation that DNA replication produces copy number variations of genomic regions, and we propose that such changes perturb metabolism. The formalization of this effect by exploiting concepts from metabolic control analysis suggests that the minimization of such perturbations during evolution could be achieved through the formation of gene clusters and operons. We support our theoretical derivations with simulations based on a realistic metabolic network, and we confirm that present-day genomes have a degree of compaction of functionally related genes, which is significantly correlated to the proposed perturbations introduced by replication. The formation of clusters of functionally related genes in microbial genomes has puzzled microbiologists since their first discovery. Here, we suggest that replication, and the copy number variations due to the replisome passage, might play a role in the process through a perturbation in metabolite homeostasis. We provide theoretical support to this hypothesis, and we found that both simulations and genomic analysis support our hypothesis. IMPORTANCE: The formation of clusters of functionally related genes in microbial genomes has puzzled microbiologists since their discovery. Here, we suggest that replication, and the copy number variations due to the replisome passage, might play a role in the process through a perturbation in metabolite homeostasis. We provide theoretical support to this hypothesis, and we found that both simulations and genomic analysis support our hypothesis.

Genetic Characterization of Rhizobium spp. Strains in an Organic Field Pea (Pisum sativum L.) Field in Lithuania.

Biological nitrogen fixation in legume plants depends on the diversity of rhizobia present in the soil. Rhizobial strains exhibit specificity towards host plants and vary in their capacity to fix nitrogen. The increasing interest in rhizobia diversity has prompted studies of their phylogenetic relations. Molecular identification of Rhizobium is quite complex, requiring multiple gene markers to be analysed to distinguish strains at the species level or to predict their host plant. In this research, 50 rhizobia isolates were obtained from the root nodules of five different Pisum sativum L. genotypes ("Bagoo", "Respect", "Astronaute", "Lina DS", and "Egle DS"). All genotypes were growing in the same field, where ecological farming practices were applied, and no commercial rhizobia inoculants were used. The influence of rhizobial isolates on pea root nodulation and dry biomass accumulation was determined. 16S rRNA gene, two housekeeping genes recA and atpD, and symbiotic gene nodC were analysed to characterize rhizobia population. The phylogenetic analysis of 16S rRNA gene sequences showed that 46 isolates were linked to Rhizobium leguminosarum; species complex 1 isolate was identified as Rhizobium nepotum, and the remaining 3 isolates belonged to Rahnella spp., Paenarthrobacter spp., and Peribacillus spp. genera. RecA and atpD gene analysis showed that the 46 isolates identified as R. leguminosarum clustered into three genospecies groups (B), (E) and (K). Isolates that had the highest influence on plant dry biomass accumulation clustered into the (B) group. NodC gene phylogenetic analysis clustered 46 R. leguminosarum isolates into 10 groups, and all isolates were assigned to the R. leguminosarum sv. viciae.

Management and outcome of oncological patients under immune checkpoint inhibitors presenting at the emergency department.

INTRODUCTION: With the rising use of immune checkpoint inhibitors (ICIs) in oncology, emergency physicians are increasingly confronted with their immune-related adverse events (irAEs). We described the types of irAEs presenting to the ED of a Belgian cancer centre and determined associations with the development of an irAE and other patient's characteristics. Secondary objectives describe the therapeutic management and determine 7 and 30-day mortality. METHODS: A retrospective chart review of ED visits of patients on ICI from 15 December 2016 to 6 December 2020 was performed. Clinical presentation, cancer characteristics and type of ICI were extracted by a single abstractor. We recorded any suspicion of irAE in the ED and confirmation of an irAE was based on the patient's oncologist report. Outcome was based on mortality at date of last follow-up. RESULTS: 227 patients on ICI presented to the ED, with a total of 451 visits. 54 (12%) of the visits resulted in a diagnosis of irAE. Four clinical features were associated with an irAE: gastrointestinal complaints (p=0.01), skin rashes (p=0.02), acute renal failure (p=0.002) and abnormal liver function (p=0.04). An irAE was also associated with three different factors: a cancer status in remission (OR=5.33, 95% CI 2.57 to 11.04), a combination of two ICIs (OR=4.43, 95% CI 2.09 to 9.42) and a medical history of irAE (OR=2.44, 95% CI 1.27 to 4.68). 30-day mortality was lower in the irAE group (0%) than in the non-irAE group (13%, 95% CI 9% to 19%). CONCLUSIONS: Oncological patients under ICI presenting in the ED are more likely to have an irAE if they present with gastrointestinal and dermatological complaints, acute renal failure and abnormal liver function. This is also true for patients with any history of irAE, a concomitant use of two ICIs and with a cancer status in remission.

The motility and chemosensory systems of Rhizobium leguminosarum, their role in symbiosis, and link to PTSNtr regulation.

Motility and chemotaxis are crucial processes for soil bacteria and plant-microbe interactions. This applies to the symbiotic bacterium Rhizobium leguminosarum, where motility is driven by flagella rotation controlled by two chemotaxis systems, Che1 and Che2. The Che1 cluster is particularly important in free-living motility prior to the establishment of the symbiosis, with a che1 mutant delayed in nodulation and reduced in nodulation competitiveness. The Che2 system alters bacteroid development and nodule maturation. In this work, we also identified 27 putative chemoreceptors encoded in the R. leguminosarum bv. viciae 3841 genome and characterized its motility in different growth conditions. We describe a metabolism-based taxis system in rhizobia that acts at high concentrations of dicarboxylates to halt motility independent of chemotaxis. Finally, we show how PTSNtr influences cell motility, with PTSNtr mutants exhibiting reduced swimming in different media. Motility is restored by the active forms of the PTSNtr output regulatory proteins, unphosphorylated ManX and phosphorylated PtsN. Overall, this work shows how rhizobia typify soil bacteria by having a high number of chemoreceptors and highlights the importance of the motility and chemotaxis mechanisms in a free-living cell in the rhizosphere, and at different stages of the symbiosis.

Flying toward a plastic-free world: Can Drosophila serve as a model organism to develop new strategies of plastic waste management?

The last century was dominated by the widespread use of plastics, both in terms of invention and increased usage. The environmental challenge we currently face is not just about reducing plastic usage but finding new ways to manage plastic waste. Recycling is growing but remains a small part of the solution. There is increasing focus on studying organisms and processes that can break down plastics, offering a modern approach to addressing the environmental crisis. Here, we provide an overview of the organisms associated with plastics biodegradation, and we explore the potential of harnessing and integrating their genetic and biochemical features into a single organism, such as Drosophila melanogaster. The remarkable genetic engineering and microbiota manipulation tools available for this organism suggest that multiple features could be amalgamated and modeled in the fruit fly. We outline feasible genetic engineering and gut microbiome engraftment strategies to develop a new class of plastic-degrading organisms and discuss of both the potential benefits and the limitations of developing such engineered Drosophila melanogaster strains.

3D Printing of Copper Using Water-Based Colloids and Reductive Sintering.

Copper was manufactured by using a low-cost 3D printing device and copper oxide water-based colloids. The proposed method avoids the use of toxic volatile solvents (used in metal-based robocasting), adopting copper oxide as a precursor of copper metal due to its lower cost and higher chemical stability. The appropriate rheological properties of the colloids have been obtained through the addition of poly-ethylene oxide-co-polypropylene-co-polyethylene oxide copolymer (Pluronic P123) and poly-acrylic acid to the suspension of the oxide in water. Mixing of the components of the colloidal suspension was performed with the same syringes used for the extrusion, avoiding any material waste. The low-temperature transition of water solutions of P123 is used to facilitate the homogenization of the colloid. The copper oxide is then converted to copper metal through a reductive sintering process, performed at 1000°C for a few hours in an atmosphere of Ar-10%H2. This approach allows the obtainment of porous copper objects (up to 20%) while retaining good mechanical properties. It could be beneficial for many applications, for example current collectors in lithium batteries.

Characterization of the microbial community in ripened Pecorino Toscano cheese affected by pink discoloration.

Pink discoloration defect can cause economic losses for cheese producers due to the impossibility to sell the defected cheese, but few knowledge is currently available on the causes of this defect. To gain more insight on the causes that lead to the formation of pink discoloration in Pecorino Toscano cheese with the Protected Designation of Origin (PDO) status, the bacterial community in defected and not defected cheese was characterized by high-throughput sequencing of bacterial 16S rRNA gene. The bacterial community in the defected cheese significantly differed compared to the control. The relative abundance of the genera Acidipropionibacterium, Enterococcus, Escherichia/Shigella, Lactobacillus, Lentilactobacillus and Propionibacterium was higher in the cheese with pink discoloration defect. The concentration of short chain fatty acids and of lactic acid in cheese was measured and a shift towards the production of propionate in the cheese with pink discoloration defect was observed. Furthermore, the possible involvement of microbially produced vitamin B12 in the formation of pink discoloration was not supported by the data, since a tendency to a lower concentration of vitamin B12 was measured in the defected cheese compared to the control.

Competitiveness for Nodule Colonization in Sinorhizobium meliloti: Combined In Vitro-Tagged Strain Competition and Genome-Wide Association Analysis.

Associations between leguminous plants and symbiotic nitrogen-fixing rhizobia are a classic example of mutualism between a eukaryotic host and a specific group of prokaryotic microbes. Although this symbiosis is in part species specific, different rhizobial strains may colonize the same nodule. Some rhizobial strains are commonly known as better competitors than others, but detailed analyses that aim to predict rhizobial competitive abilities based on genomes are still scarce. Here, we performed a bacterial genome-wide association (GWAS) analysis to define the genomic determinants related to the competitive capabilities in the model rhizobial species Sinorhizobium meliloti. For this, 13 tester strains were green fluorescent protein (GFP) tagged and assayed versus 3 red fluorescent protein (RFP)-tagged reference competitor strains (Rm1021, AK83, and BL225C) in a Medicago sativa nodule occupancy test. Competition data and strain genomic sequences were employed to build a model for GWAS based on k-mers. Among the k-mers with the highest scores, 51 k-mers mapped on the genomes of four strains showing the highest competition phenotypes (>60% single strain nodule occupancy; GR4, KH35c, KH46, and SM11) versus BL225C. These k-mers were mainly located on the symbiosis-related megaplasmid pSymA, specifically on genes coding for transporters, proteins involved in the biosynthesis of cofactors, and proteins related to metabolism (e.g., fatty acids). The same analysis was performed considering the sum of single and mixed nodules obtained in the competition assays versus BL225C, retrieving k-mers mapped on the genes previously found and on vir genes. Therefore, the competition abilities seem to be linked to multiple genetic determinants and comprise several cellular components. IMPORTANCE Decoding the competitive pattern that occurs in the rhizosphere is challenging in the study of bacterial social interaction strategies. To date, the single-gene approach has mainly been used to uncover the bases of nodulation, but there is still a knowledge gap regarding the main features that a priori characterize rhizobial strains able to outcompete indigenous rhizobia. Therefore, tracking down which traits make different rhizobial strains able to win the competition for plant infection over other indigenous rhizobia will improve the strain selection process and, consequently, plant yield in sustainable agricultural production systems. We proved that a k-mer-based GWAS approach can efficiently identify the competition determinants of a panel of strains previously analyzed for their plant tissue occupancy using double fluorescent labeling. The reported strategy will be useful for detailed studies on the genomic aspects of the evolution of bacterial symbiosis and for an extensive evaluation of rhizobial inoculants.

Global control of bacterial nitrogen and carbon metabolism by a PTSNtr-regulated switch.

The nitrogen-related phosphotransferase system (PTSNtr) of Rhizobium leguminosarum bv. viciae 3841 transfers phosphate from PEP via PtsP and NPr to two output regulators, ManX and PtsN. ManX controls central carbon metabolism via the tricarboxylic acid (TCA) cycle, while PtsN controls nitrogen uptake, exopolysaccharide production, and potassium homeostasis, each of which is critical for cellular adaptation and survival. Cellular nitrogen status modulates phosphorylation when glutamine, an abundant amino acid when nitrogen is available, binds to the GAF sensory domain of PtsP, preventing PtsP phosphorylation and subsequent modification of ManX and PtsN. Under nitrogen-rich, carbon-limiting conditions, unphosphorylated ManX stimulates the TCA cycle and carbon oxidation, while unphosphorylated PtsN stimulates potassium uptake. The effects are reversed with the phosphorylation of ManX and PtsN, occurring under nitrogen-limiting, carbon-rich conditions; phosphorylated PtsN triggers uptake and nitrogen metabolism, the TCA cycle and carbon oxidation are decreased, while carbon-storage polymers such as surface polysaccharide are increased. Deleting the GAF domain from PtsP makes cells "blind" to the cellular nitrogen status. PTSNtr constitutes a switch through which carbon and nitrogen metabolism are rapidly, and reversibly, regulated by protein:protein interactions. PTSNtr is widely conserved in proteobacteria, highlighting its global importance.

Characterization of the microbial community composition in Italian Cinta Senese sausages dry-fermented with natural extracts as alternatives to sodium nitrite.

Nitrite is widely used in meat products as a multifunctional additive, combining flavour and colour properties with antioxidant and antimicrobial effects. However, nitrite may form reaction products (i.e., nitrosamine) that are potentially carcinogenic to humans. The meat industry, in response to consumers' demands for nitrite-free products, is seeking natural alternatives to nitrite, such as plant-based extracts. Three types of dry-fermented sausages were manufactured: NIT, containing 30 ppm of sodium nitrite; GSE, containing grape seed extract and olive pomace hydroxytyrosol; and CHE, containing chestnut extract and olive pomace hydroxytyrosol. Next-generation sequencing (NGS) was used to analyse microbial consortia, which were correlated with physical and chemical parameters. The prokaryotic community composition was similar among treatments, with a high relative abundance of Staphylococcus xylosus and Lactobacillus sakei, collectively accounting for 87% of the total community. However, significant differences were observed in both operational taxonomic unit (OTU) presence/absence and relative abundance. Ten genera varied in abundance between treatments. The increase in Lactobacillaceae in CHE may explain the reduced pH levels detected in these samples. In conclusion, NGS analysis showed that the prokaryotic community composition was similar in GSE and NIT, while CHE varied in both the composition and relative abundance of different taxa.

Agricultural Selection of Wheat Has Been Shaped by Plant-Microbe Interactions.

The influence of wheat (modern wheat, both bread and pasta, their wild ancestors and synthetic hybrids) on the microbiota of their roots and surrounding soil is characterized. We isolated lines of bread wheat by hybridizing diploid (Aegilops tauschii) with tetraploid Triticum durum and crossed it with a modern cultivar of Triticum aestivum. The newly created, synthetic hybrid wheat, which recapitulate the breeding history of wheat through artificial selection, is found to support a microbiome enriched in beneficial Glomeromycetes fungi, but also in, potentially detrimental, Nematoda. We hypothesize that during wheat domestication this plant-microbe interaction diminished, suggesting an evolutionary tradeoff; sacrificing advantageous nutrient acquisition through fungal interactions to minimize interaction with pathogenic fungi. Increased plant selection for Glomeromycetes and Nematoda is correlated with the D genome derived from A. tauschii. Despite differences in their soil microbiota communities, overall wheat plants consistently show a low ratio of eukaryotes to prokaryotes. We propose that this is a mechanism for protection against soil-borne fungal disease and appears to be deeply rooted in the wheat genome. We suggest that the influence of plants on the composition of their associated microbiota is an integral factor, hitherto overlooked, but intrinsic to selection during wheat domestication.

Methylation-dependent transcriptional regulation of crescentin gene (creS) by GcrA in Caulobacter crescentus.

In Caulobacter crescentus the combined action of chromosome replication and the expression of DNA methyl-transferase CcrM at the end of S-phase maintains a cyclic alternation between a full- to hemi-methylated chromosome. This transition of the chromosomal methylation pattern affects the DNA-binding properties of the transcription factor GcrA that controls the several key cell cycle functions. However, the molecular mechanism by which GcrA and methylation are linked to transcription is not fully elucidated yet. Using a combination of cell biology, genetics, and in vitro analysis, we deciphered how GcrA integrates the methylation pattern of several S-phase expressed genes to their transcriptional output. We demonstrated in vitro that transcription of ctrA from the P1 promoter in its hemi-methylated state is activated by GcrA, while in its fully methylated state GcrA had no effect. Further, GcrA and methylation together influence a peculiar distribution of creS transcripts, encoding for crescentin, the protein responsible for the characteristic shape of Caulobacter cells. This gene is duplicated at the onset of chromosome replication and the two hemi-methylated copies are spatially segregated. Our results indicated that GcrA transcribed only the copy where coding strand is methylated. In vitro transcription assay further substantiated this finding. As several of the cell cycle-regulated genes are also under the influence of methylation and GcrA-dependent transcriptional regulation, this could be a mechanism responsible for maintaining the gene transcription dosage during the S-phase.

The Degradative Capabilities of New Amycolatopsis Isolates on Polylactic Acid.

Polylactic acid (PLA), a bioplastic synthesized from lactic acid, has a broad range of applications owing to its excellent proprieties such as a high melting point, good mechanical strength, transparency, and ease of fabrication. However, the safe disposal of PLA is an emerging environmental problem: it resists microbial attack in environmental conditions, and the frequency of PLA-degrading microorganisms in soil is very low. To date, a limited number of PLA-degrading bacteria have been isolated, and most are actinomycetes. In this work, a method for the selection of rare actinomycetes with extracellular proteolytic activity was established, and the technique was used to isolate four mesophilic actinomycetes with the ability to degrade emulsified PLA in agar plates. All four strains-designated SO1.1, SO1.2, SNC, and SST-belong to the genus Amycolatopsis. The PLA-degrading capability of the four strains was investigated by testing their ability to assimilate lactic acid, fragment PLA polymers, and deteriorate PLA films. The strain SNC was the best PLA degrader-it was able to assimilate lactic acid, constitutively cleave PLA, and form a thick and widespread biofilm on PLA film. The activity of this strain extensively eroded the polymer, leading to a weight loss of 36% in one month in mesophilic conditions.

Harnessing Rhizobia to Improve Heavy-Metal Phytoremediation by Legumes.

Rhizobia are bacteria that can form symbiotic associations with plants of the Fabaceae family, during which they reduce atmospheric di-nitrogen to ammonia. The symbiosis between rhizobia and leguminous plants is a fundamental contributor to nitrogen cycling in natural and agricultural ecosystems. Rhizobial microsymbionts are a major reason why legumes can colonize marginal lands and nitrogen-deficient soils. Several leguminous species have been found in metal-contaminated areas, and they often harbor metal-tolerant rhizobia. In recent years, there have been numerous efforts and discoveries related to the genetic determinants of metal resistance by rhizobia, and on the effectiveness of such rhizobia to increase the metal tolerance of host plants. Here, we review the main findings on the metal resistance of rhizobia: the physiological role, evolution, and genetic determinants, and the potential to use native and genetically-manipulated rhizobia as inoculants for legumes in phytoremediation practices.

Effect of different types of olive oil pomace dietary supplementation on the rumen microbial community profile in Comisana ewes.

Olive oil pomace (OOP) is a bio-waste rich in highly soluble polyphenols. OOP has been proposed as an additive in ruminant feeding to modulate rumen fermentations. Three groups of ewes were fed the following different diets: a control diet and two diets supplemented with OOP, obtained with a two-phase (OOP2) or three-phase (OOP3) olive milling process. Rumen liquor (RL) showed a higher content of 18:3 cis9 cis12 cis15 (α-linolenic acid, α-LNA) with OOP2 inclusion, and of 18:2 cis9 trans11 (rumenic acid, RA) with OOP3 inclusion. The overall composition of the RL microbiota did not differ among treatments. Significant differences, between control and treated groups, were found for six bacterial taxa. In particular, RL microbiota from animals fed OOPs showed a reduction in Anaerovibrio, a lipase-producing bacterium. The decrease in the Anaerovibrio genus may lead to a reduction in lipolysis, thus lowering the amount of polyunsaturated fatty acids available for biohydrogenation. Milk from animals fed OOP showed a higher content of 18:1 cis9 (oleic acid, OA) but the α-LNA concentration was increased in milk from animals treated with OOP2 only. Therefore, inclusion of OOP in ruminant diets may be a tool to ameliorate the nutritional characteristics of milk.

Bacterial Biosensors for in Vivo Spatiotemporal Mapping of Root Secretion.

Plants engineer the rhizosphere to their advantage by secreting various nutrients and secondary metabolites. Coupling transcriptomic and metabolomic analyses of the pea (Pisum sativum) rhizosphere, a suite of bioreporters has been developed in Rhizobium leguminosarum bv viciae strain 3841, and these detect metabolites secreted by roots in space and time. Fourteen bacterial lux fusion bioreporters, specific for sugars, polyols, amino acids, organic acids, or flavonoids, have been validated in vitro and in vivo. Using different bacterial mutants (nodC and nifH), the process of colonization and symbiosis has been analyzed, revealing compounds important in the different steps of the rhizobium-legume association. Dicarboxylates and sucrose are the main carbon sources within the nodules; in ineffective (nifH) nodules, particularly low levels of sucrose were observed, suggesting that plant sanctions affect carbon supply to nodules. In contrast, high myo-inositol levels were observed prior to nodule formation and also in nifH senescent nodules. Amino acid biosensors showed different patterns: a γ-aminobutyrate biosensor was active only inside nodules, whereas the phenylalanine bioreporter showed a high signal also in the rhizosphere. The bioreporters were further validated in vetch (Vicia hirsuta), producing similar results. In addition, vetch exhibited a local increase of nod gene-inducing flavonoids at sites where nodules developed subsequently. These bioreporters will be particularly helpful in understanding the dynamics of root exudation and the role of different molecules secreted into the rhizosphere.

Evolution of Intra-specific Regulatory Networks in a Multipartite Bacterial Genome.

Reconstruction of the regulatory network is an important step in understanding how organisms control the expression of gene products and therefore phenotypes. Recent studies have pointed out the importance of regulatory network plasticity in bacterial adaptation and evolution. The evolution of such networks within and outside the species boundary is however still obscure. Sinorhizobium meliloti is an ideal species for such study, having three large replicons, many genomes available and a significant knowledge of its transcription factors (TF). Each replicon has a specific functional and evolutionary mark; which might also emerge from the analysis of their regulatory signatures. Here we have studied the plasticity of the regulatory network within and outside the S. meliloti species, looking for the presence of 41 TFs binding motifs in 51 strains and 5 related rhizobial species. We have detected a preference of several TFs for one of the three replicons, and the function of regulated genes was found to be in accordance with the overall replicon functional signature: house-keeping functions for the chromosome, metabolism for the chromid, symbiosis for the megaplasmid. This therefore suggests a replicon-specific wiring of the regulatory network in the S. meliloti species. At the same time a significant part of the predicted regulatory network is shared between the chromosome and the chromid, thus adding an additional layer by which the chromid integrates itself in the core genome. Furthermore, the regulatory network distance was found to be correlated with both promoter regions and accessory genome evolution inside the species, indicating that both pangenome compartments are involved in the regulatory network evolution. We also observed that genes which are not included in the species regulatory network are more likely to belong to the accessory genome, indicating that regulatory interactions should also be considered to predict gene conservation in bacterial pangenomes.

Cell Cycle Control by the Master Regulator CtrA in Sinorhizobium meliloti.

In all domains of life, proper regulation of the cell cycle is critical to coordinate genome replication, segregation and cell division. In some groups of bacteria, e.g. Alphaproteobacteria, tight regulation of the cell cycle is also necessary for the morphological and functional differentiation of cells. Sinorhizobium meliloti is an alphaproteobacterium that forms an economically and ecologically important nitrogen-fixing symbiosis with specific legume hosts. During this symbiosis S. meliloti undergoes an elaborate cellular differentiation within host root cells. The differentiation of S. meliloti results in massive amplification of the genome, cell branching and/or elongation, and loss of reproductive capacity. In Caulobacter crescentus, cellular differentiation is tightly linked to the cell cycle via the activity of the master regulator CtrA, and recent research in S. meliloti suggests that CtrA might also be key to cellular differentiation during symbiosis. However, the regulatory circuit driving cell cycle progression in S. meliloti is not well characterized in both the free-living and symbiotic state. Here, we investigated the regulation and function of CtrA in S. meliloti. We demonstrated that depletion of CtrA cause cell elongation, branching and genome amplification, similar to that observed in nitrogen-fixing bacteroids. We also showed that the cell cycle regulated proteolytic degradation of CtrA is essential in S. meliloti, suggesting a possible mechanism of CtrA depletion in differentiated bacteroids. Using a combination of ChIP-Seq and gene expression microarray analysis we found that although S. meliloti CtrA regulates similar processes as C. crescentus CtrA, it does so through different target genes. For example, our data suggest that CtrA does not control the expression of the Fts complex to control the timing of cell division during the cell cycle, but instead it negatively regulates the septum-inhibiting Min system. Our findings provide valuable insight into how highly conserved genetic networks can evolve, possibly to fit the diverse lifestyles of different bacteria.

The DivJ, CbrA and PleC system controls DivK phosphorylation and symbiosis in Sinorhizobium meliloti.

Sinorhizobium meliloti is a soil bacterium that invades the root nodules it induces on Medicago sativa, whereupon it undergoes an alteration of its cell cycle and differentiates into nitrogen-fixing, elongated and polyploid bacteroid with higher membrane permeability. In Caulobacter crescentus, a related alphaproteobacterium, the principal cell cycle regulator, CtrA, is inhibited by the phosphorylated response regulator DivK. The phosphorylation of DivK depends on the histidine kinase DivJ, while PleC is the principal phosphatase for DivK. Despite the importance of the DivJ in C. crescentus, the mechanistic role of this kinase has never been elucidated in other Alphaproteobacteria. We show here that the histidine kinases DivJ together with CbrA and PleC participate in a complex phosphorylation system of the essential response regulator DivK in S. meliloti. In particular, DivJ and CbrA are involved in DivK phosphorylation and in turn CtrA inactivation, thereby controlling correct cell cycle progression and the integrity of the cell envelope. In contrast, the essential PleC presumably acts as a phosphatase of DivK. Interestingly, we found that a DivJ mutant is able to elicit nodules and enter plant cells, but fails to establish an effective symbiosis suggesting that proper envelope and/or low CtrA levels are required for symbiosis.