Concomitant Loss of the Glyoxalase System and Glycolysis Makes the Uncultured Pathogen Candidatus Liberibacter asiaticus an Energy Scavenger

Methylglyoxal (MG) is a cytotoxic, nonenzymatic by-product of glycolysis that readily glycates proteins and DNA, resulting in carbonyl stress. Glyoxalase I and II (GloA and GloB) sequentially convert MG into D-lactic acid using glutathione (GSH) as a cofactor. The glyoxalase system is essential for the mitigation of MG-induced carbonyl stress, preventing subsequent cell death, and recycling GSH for maintenance of cellular redox poise. All pathogenic liberibacters identified to date are uncultured, including Candidatus Liberibacter asiaticus, a psyllid endosymbiont and causal agent of the severely damaging citrus disease huanglongbing. In silico analysis revealed the absence of gloA in Ca. Liberibacter asiaticus and all other pathogenic liberibacters. Both gloA and gloB are present in Liberibacter crescens, the only liberibacter that has been cultured. L. crescens GloA was functional in a heterologous host. Marker interruption of gloA in L. crescens appeared to be lethal. Key glycolytic enzymes were either missing or significantly downregulated in Ca. Liberibacter asiaticus compared to (cultured) L. crescens. Marker interruption of sut, a sucrose transporter gene in L. crescens, decreased its ability to take up exogenously supplied sucrose in culture. Ca. Liberibacter asiaticus lacks a homologous sugar transporter but has a functional ATP/ADP translocase, enabling it to thrive both in psyllids and in the sugar-rich citrus phloem by (i) avoiding sucrose uptake, (ii) avoiding MG generation via glycolysis, and (iii) directly importing ATP from the host cell. MG detoxification enzymes appear to be predictive of Candidatus status for many uncultured pathogenic and environmental bacteria. IMPORTANCE Discovered more than 100 years ago, the glyoxalase system is thought to be present across all domains of life and fundamental to cellular growth and viability. The glyoxalase system protects against carbonyl stress caused by methylglyoxal (MG), a highly reactive, mutagenic and cytotoxic compound that is nonenzymatically formed as a by-product of glycolysis. The uncultured alphaproteobacterium Ca. Liberibacter asiaticus is a well-adapted endosymbiont of the Asian citrus psyllid, which transmits the severely damaging citrus disease huanglongbing. Ca. Liberibacter asiaticus lacks a functional glyoxalase pathway. We report here that the bacterium is able to thrive both in psyllids and in the sugar-rich citrus phloem by (i) avoiding sucrose uptake, (ii) avoiding (significant) MG generation via glycolysis, and (iii) directly importing ATP from the host cell. We hypothesize that failure to culture Ca. Liberibacter asiaticus is at least partly due to its dependence on host cells for both ATP and MG detoxification.