ABSTRACT
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.