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Phytoplasmas ('Candidatus Phytoplasma') are insect-vectored plant pathogens. The genomes of these bacteria are small with limited metabolic capacities making them dependent on their plant and insect hosts for survival. In contrast to mycoplasmas and other relatives in the class Mollicutes, phytoplasmas encode genes for malate transporters and malic enzyme (ME) for conversion of malate into pyruvate. It was hypothesized that malate is probably a major energy source for phytoplasmas as these bacteria are limited in the uptake and processing of carbohydrates. In this study, we investigated the metabolic capabilities of 'Candidatus (Ca.) phytoplasma' aster yellows witches'-broom (AYWB) malic enzyme (ME). We found that AYWB-ME has malate oxidative decarboxylation activity, being able to convert malate to pyruvate and CO2 with the reduction of either NAD or NADP, and displays distinctive kinetic mechanisms depending on the relative concentration of the substrates. AYWB-ME activity was strictly modulated by the ATP/ADP ratio, a feature which has not been found in other ME isoforms characterized to date. In addition, we found that the 'Ca. Phytoplasma' AYWB PduL-like enzyme (AYWB-PduL) harbours phosphotransacetylase activity, being able to convert acetyl-CoA to acetyl phosphate downstream of pyruvate. ATP also inhibited AYWB-PduL activity, as with AYWB-ME, and the product of the reaction catalysed by AYWB-PduL, acetyl phosphate, stimulated AYWB-ME activity. Overall, our data indicate that AYWB-ME and AYWB-PduL activities are finely coordinated by common metabolic signals, like ATP/ADP ratios and acetyl phosphate, which support their participation in energy (ATP) and reducing power [NAD(P)H] generation from malate in phytoplasmas.

Metabolic regulation of phytoplasma malic enzyme and phosphotransacetylase supports the use of malate as an energy source in these plant pathogens