Transcriptional regulation and mutational analysis of a dctA gene encoding an organic acid transporter protein from Pseudomonas chlororaphis O6

The phage-encoded PIT4 protein affects multiple two-component systems of Pseudomonas aeruginosa

IMPORTANCE

More and more Pseudomonas aeruginosa isolates have become resistant to antibiotics like carbapenem. As a consequence, P. aeruginosa ranks in the top three of pathogens for which the development of novel antibiotics is the most crucial. The pathogen causes both acute and chronic infections, especially in patients who are the most vulnerable. Therefore, efforts are urgently needed to develop alternative therapies. One path explored in this article is the use of bacteriophages and, more specifically, phage-derived proteins. In this study, a phage-derived protein was studied that impacts key virulence factors of the pathogen via interaction with multiple histidine kinases of TCSs. The fundamental insights gained for this protein can therefore serve as inspiration for the development of an anti-virulence compound that targets the bacterial TCS.

Tetracarboxylic acid transporter regulates growth, conidiation, and carbon utilization in Metarhizium acridum

Carbon sources and their utilization are vital for fungal growth and development. C₄-dicarboxylic acids are important carbon and energy sources that function as intermediate products of the tricarboxylic acid cycle. Transport and regulation of C₄-dicarboxylic acid uptake are mainly dependent on tetracarboxylic acid transporters (Dcts) in many microbes, although the roles of Dct genes in fungi have only been partially characterized. Here, we report on the functions of two Dct genes (Dct1 and Dct2) in the entomopathogenic fungus Metarhizium acridum. Our data showed that loss of the MaDct1 gene affected utilization of tetracarboxylic acids and other carbon sources. ΔMaDct1 mutants showed larger colony sizes with extensive mycelial growth but were delayed in conidiation with decreased conidia yield as compared to the wild-type parental strain. On the nutrient-deficient medium, SYA, the wild-type strain produced microcycle conidia, whereas the ΔMaDct1 mutant produced (normal) aerial conidia. In addition, ΔMaDct1 had decreased tolerance to cell wall perturbing agents, but increased tolerances to UV-B radiation and osmotic stress. Insect bioassays indicated that loss of MaDct1 did not affect pathogenicity. In contrast, no distinct phenotypic change was observed for the MaDct2 mutant in terms of growth and biocontrol characteristics. Transcriptomic profiling between wild type and ΔMaDct1 showed that differentially expressed genes were enriched in carbohydrate and amino acid metabolism, transport and catabolism, and signal transduction. These results demonstrate that MaDct1 regulates the conidiation pattern shift and mycelial growth by affecting utilization of carbon sources. These findings are helpful for better understanding the effect of intermediates of carbon metabolism on fungal growth and conidiation. KEY POINTS: • MaDct1 influences fungal growth and conidiation by affecting carbon source utilization. • MaDct1 regulates conidiation pattern shift under nutrient deficiency condition. • MaDct1 is involved in stress tolerance and has no effect on virulence. • MaDct2 has no effect on growth and biocontrol characteristic.

Lignin induced iron reduction by novel sp., Tolumonas lignolytic BRL6-1

Lignin is the second most abundant carbon polymer on earth and despite having more fuel value than cellulose, it currently is considered a waste byproduct in many industrial lignocellulose applications. Valorization of lignin relies on effective and green methods of de-lignification, with a growing interest in the use of microbes. Here we investigate the physiology and molecular response of the novel facultative anaerobic bacterium, Tolumonas lignolytica BRL6-1, to lignin under anoxic conditions. Physiological and biochemical changes were compared between cells grown anaerobically in either lignin-amended or unamended conditions. In the presence of lignin, BRL6-1 accumulates higher biomass and has a shorter lag phase compared to unamended conditions, and 14% of the proteins determined to be significantly higher in abundance by log2 fold-change of 2 or greater were related to Fe(II) transport in late logarithmic phase. Ferrozine assays of the supernatant confirmed that Fe(III) was bound to lignin and reduced to Fe(II) only in the presence of BRL6-1, suggesting redox activity by the cells. LC-MS/MS analysis of the secretome showed an extra band at 20 kDa in lignin-amended conditions. Protein sequencing of this band identified a protein of unknown function with homology to enzymes in the radical SAM superfamily. Expression of this protein in lignin-amended conditions suggests its role in radical formation. From our findings, we suggest that BRL6-1 is using a protein in the radical SAM superfamily to interact with the Fe(III) bound to lignin and reducing it to Fe(II) for cellular use, increasing BRL6-1 yield under lignin-amended conditions. This interaction potentially generates organic free radicals and causes a radical cascade which could modify and depolymerize lignin. Further research should clarify the extent to which this mechanism is similar to previously described aerobic chelator-mediated Fenton chemistry or radical producing lignolytic enzymes, such as lignin peroxidases, but under anoxic conditions.

Identification of C(4)-dicarboxylate transport systems in Pseudomonas aeruginosa PAO1

Pseudomonas aeruginosa utilizes preferentially C(4)-dicarboxylates such as malate, fumarate, and succinate as carbon and energy sources. We have identified and characterized two C(4)-dicarboxylate transport (Dct) systems in P. aeruginosa PAO1. Inactivation of the dctA(PA1183) gene caused a growth defect of the strain in minimal media supplemented with succinate, fumarate or malate, indicating that DctA has a major role in Dct. However, residual growth of the dctA mutant in these media suggested the presence of additional C(4)-dicarboxylate transporter(s). Tn5 insertion mutagenesis of the ΔdctA mutant led to the identification of a second Dct system, i.e., the DctPQM transporter belonging to the tripartite ATP-independent periplasmic (TRAP) family of carriers. The ΔdctA ΔdctPQM double mutant showed no growth on malate and fumarate and residual growth on succinate, suggesting that DctA and DctPQM are the only malate and fumarate transporters, whereas additional transporters for succinate are present. Using lacZ reporter fusions, we showed that the expression of the dctA gene and the dctPQM operon was enhanced in early exponential growth phase and induced by C(4)-dicarboxylates. Competition experiments demonstrated that the DctPQM carrier was more efficient than the DctA carrier for the utilization of succinate at micromolar concentrations, whereas DctA was the major transporter at millimolar concentrations. To conclude, this is the first time that the high- and low-affinity uptake systems for succinate DctA and DctPQM have been reported to function coordinately to transport C(4)-dicarboxylates and that the alternative sigma factor RpoN and a DctB/DctD two-component system regulates simultaneously the dctA gene and the dctPQM operon.

Mutation in the edd gene encoding the 6-phosphogluconate dehydratase of Pseudomonas chlororaphis O6 impairs root colonization and is correlated with reduced induction of systemic resistance

AIMS

The primary objective of this study was to determine the role of 6-phosphogluconate dehydratase in root colonization and the induction of systemic resistance by the rhizobacterium, Pseudomonas chlororaphis O6.

METHODS AND RESULTS

The edd gene encoding for 6-phosphogluconate dehydratase, which is one of the key enzymes in glucose utilization, was cloned. Transcription of the gene was higher in medium containing sugars than with organic acids. An edd mutant failed to grow on glucose but grew on organic acids. The edd mutant colonized tobacco roots at wild-type levels early after inoculation, but levels were lower by 12 days. The edd mutant failed to induce the systemic resistance in tobacco to a soft-rot pathogen at wild-type level.

CONCLUSIONS

6-Phosphogluconate dehydratase in P. chlororaphis O6 contributes to root colonization and induction of systemic resistance presumably as the consequence of its essential role in the Entner-Doudoroff (ED) pathway.

SIGNIFICANCE AND IMPACT OF THE STUDY

Metabolism of sugars through the ED pathway in P. chlororaphis O6 may be important because it facilitates the production of inducers of systemic resistance including butanediol.

The dctA gene of Pseudomonas chlororaphis O6 is under RpoN control and is required for effective root colonization and induction of systemic resistance

Transcription from the dctA gene, which encodes an organic acid transporter in the root-colonizing bacterium Pseudomonas chlororaphis O6, is under complex regulatory control. Promoter sequence analysis revealed an RpoN binding site. The regulation of transcript accumulation by the level of ammonium ions in the growth medium confirmed RpoN regulation, even in the presence of glucose. A dctA mutant colonized tobacco roots to a lesser extent than the wild-type mutant during early seedling development. Colonization by the dctA mutant, as compared to the wild type, also reduced the level of systemically induced resistance against the soft rot pathogen Erwinia carotovora SCC1. We ascribe this reduced colonization to the inability of the mutant to utilize certain organic acid components in the root exudates. The dctA mutant failed to grow on succinate and fumarate, and showed reduced growth on malate. All altered properties of the mutant were complemented by the full-length dctA gene. We propose that organic acids in root exudates may provide important nutrient sources for the beneficial root-colonizing pseudomonad.