Genetic Regulation of Alginate Production in Azotobacter vinelandii a Bacterium of Biotechnological Interest: A Mini-Review

The c-di-GMP effector FleQ controls alginate production by repressing transcription of algD in Azotobacter vinelandii

Production of the exopolysaccharide alginate by Azotobacter vinelandii, member of the Pseudomonadaceae family, is positively controlled by the second messenger c-di-GMP. This effect was solely attributed to the role of c-di-GMP in activating the alginate polymerase complex. In this study, the role of c-di-GMP in algD transcription, which encodes the key enzyme for alginate synthesis, was investigated. algD transcription correlated with artificially high or low levels of c-di-GMP. Moreover, FleQ, one of the best-characterized c-di-GMP effectors, was found to exert a negative effect on alginate production and algD transcription, as both increased in a ΔfleQ mutant relative to the wild-type strain or the ΔfleQ/fleQ+ complemented strain. Electrophoretic mobility shift assays (EMSAs) confirmed that FleQ directly binds to the regulatory region of algD, which was consistent with the presence of two FleQ binding sites in the vicinity of the algD RpoS-dependent promoter. In A. vinelandii, c-di-GMP is essential for the expression of alginate C-5 epimerases (AlgE1-6), which are necessary for structuring the envelope of differentiated cells, known as cysts. However, FleQ was not involved in this regulation. Collectively, our results support a model in which algD transcription is under the positive control of c-di-GMP, while FleQ may only partially mediate this effect. In contrast, our study revealed a FleQ-independent regulatory mechanism for the control of A. vinelandii encystment.

In vitro and in vivo studies of selenium nanoparticles coated bacterial polysaccharide as anti-lung cancer agents

Microbial exopolysaccharides (EPSs) possess valuable biological functions and fascinating physicochemical properties. On the other hand, lung cancer is the primary contributor to global cancer-related deaths. However, health and safety concerns have prevented the identification and approval of any medications, including chemotherapeutic agents, for lung cancer treatment to date. The current study aims to enhance the production of bacterial EPS as a coating agent for the synthesis of selenium nanoparticles (AZEPS-SeNPs), to enhance their biological activity against pathogenic microbes, human lung adenocarcinoma cells (A549) in vitro, and diethyl nitrosamine (DEN)-induced lung cancer in vivo. The synthesized AZEPS-SeNPs exhibited a significant antifungal effect reaching 49.3 mm against Candida albicans. SeNPs and EPSs demonstrated a concentration-dependent synergistic antioxidant effect of 96.8%. Moreover, the synthesized nanoparticles showed a highly potent cytotoxic effect against A549 cells (1.724 ± 0.08 µg/mL) with a therapeutic index of 7.18 ± 0.21 that leads to increased reactive oxygen species (ROS) production. AZEPS-SeNPs demonstrated a proapoptotic effect on the lung adenocarcinoma A549 cell line by stimulating caspase 3 and Bax (7.08-fold and 6.505-fold, respectively), inhibiting the anti-apoptotic gene Bcl2, and arresting the cell cycle in the S phase. In vivo study revealed that the AZEPS-SeNPs-treated group showed improved histopathological examination of lung tissue sections. The present study concluded the efficiency of the synthesized bacterial EPS-SeNPs as multi-functional antimicrobial, anticancer and antioxidant agent.

A Homolog of the Histidine Kinase RetS Controls the Synthesis of Alginates, PHB, Alkylresorcinols, and Motility in Azotobacter vinelandii

The two-component system GacS/A and the posttranscriptional control system Rsm constitute a genetic regulation pathway in Gammaproteobacteria; in some species of Pseudomonas, this pathway is part of a multikinase network (MKN) that regulates the activity of the Rsm system. In this network, the activity of GacS is controlled by other kinases. One of the most studied MKNs is the MKN-GacS of Pseudomonas aeruginosa, where GacS is controlled by the kinases RetS and LadS; RetS decreases the kinase activity of GacS, whereas LadS stimulates the activity of the central kinase GacS. Outside of the Pseudomonas genus, the network has been studied only in Azotobacter vinelandii. In this work, we report the study of the RetS kinase of A. vinelandii; as expected, the phenotypes affected in gacS mutants, such as production of alginates, polyhydroxybutyrate, and alkylresorcinols and swimming motility, were also affected in retS mutants. Interestingly, our data indicated that RetS in A. vinelandii acts as a positive regulator of GacA activity. Consistent with this finding, mutation in retS also negatively affected the expression of small regulatory RNAs belonging to the Rsm family. We also confirmed the interaction of RetS with GacS, as well as with the phosphotransfer protein HptB.

Structural basis for the minimal bifunctional alginate epimerase AlgE3 from Azotobacter chroococcum

Among the epimerases specific to alginate, some of them in Azotobacter genera convert β-d-mannuronic acid to α-l-guluronic acid but also have lyase activity to degrade alginate. The remarkable characteristics of these epimerases make it a promising enzyme for tailoring alginates to meet specific demands. Here, we determined the structure of the bifunctional mannuronan C-5 epimerase AlgE3 from Azotobacter chroococcum (AcAlgE3) in complex with several mannuronic acid oligomers as well as in apo form, which allowed us to elucidate the binding manner of each mannuronic acid oligomer, and the structural plasticity, which is dependent on calcium ions. Moreover, a comprehensive analysis of the lyase activity profiles of AcAlgE3 combined with structural characteristics explained the preference for different chain length oligomers.

The Azotobacter vinelandii AlgU regulon during vegetative growth and encysting conditions: A proteomic approach

In the Pseduomonadacea family, the extracytoplasmic function sigma factor AlgU is crucial to withstand adverse conditions. Azotobacter vinelandii, a closed relative of Pseudomonas aeruginosa, has been a model for cellular differentiation in Gram-negative bacteria since it forms desiccation-resistant cysts. Previous work demonstrated the essential role of AlgU to withstand oxidative stress and on A. vinelandii differentiation, particularly for the positive control of alginate production. In this study, the AlgU regulon was dissected by a proteomic approach under vegetative growing conditions and upon encystment induction. Our results revealed several molecular targets that explained the requirement of this sigma factor during oxidative stress and extended its role in alginate production. Furthermore, we demonstrate that AlgU was necessary to produce alkyl resorcinols, a type of aromatic lipids that conform the cell membrane of the differentiated cell. AlgU was also found to positively regulate stress resistance proteins such as OsmC, LEA-1, or proteins involved in trehalose synthesis. A position-specific scoring-matrix (PSSM) was generated based on the consensus sequence recognized by AlgU in P. aeruginosa, which allowed the identification of direct AlgU targets in the A. vinelandii genome. This work further expands our knowledge about the function of the ECF sigma factor AlgU in A. vinelandii and contributes to explains its key regulatory role under adverse conditions.

Holistic view of biological nitrogen fixation and phosphorus mobilization in Azotobacter chroococcum NCIMB 8003

Nitrogen (N) and phosphorus (P) deficiencies are two of the most agronomic problems that cause significant decrease in crop yield and quality. N and P chemical fertilizers are widely used in current agriculture, causing environmental problems and increasing production costs. Therefore, the development of alternative strategies to reduce the use of chemical fertilizers while maintaining N and P inputs are being investigated. Although dinitrogen is an abundant gas in the atmosphere, it requires biological nitrogen fixation (BNF) to be transformed into ammonium, a nitrogen source assimilable by living organisms. This process is bioenergetically expensive and, therefore, highly regulated. Factors like availability of other essential elements, as phosphorus, strongly influence BNF. However, the molecular mechanisms of these interactions are unclear. In this work, a physiological characterization of BNF and phosphorus mobilization (PM) from an insoluble form (Ca₃(PO₄)₂) in Azotobacter chroococcum NCIMB 8003 was carried out. These processes were analyzed by quantitative proteomics in order to detect their molecular requirements and interactions. BNF led to a metabolic change beyond the proteins strictly necessary to carry out the process, including the metabolism related to other elements, like phosphorus. Also, changes in cell mobility, heme group synthesis and oxidative stress responses were observed. This study also revealed two phosphatases that seem to have the main role in PM, an exopolyphosphatase and a non-specific alkaline phosphatase PhoX. When both BNF and PM processes take place simultaneously, the synthesis of nitrogenous bases and L-methionine were also affected. Thus, although the interdependence is still unknown, possible biotechnological applications of these processes should take into account the indicated factors.