K. Matsuno et al., Metabolic imbalance and sporulation in an isocitrate dehydrogenase mutant of Bacillus subtilis, J BACT, 181(11), 1999, pp. 3382-3391
A Bacillus subtilis mutant with a deletion in the citC gene, encoding isoci
trate dehydrogenase, the third enzyme of the tricarboxylic acid branch of t
he Krebs cycle, exhibited reduced growth yield in broth medium and had grea
tly reduced ability to sporulate compared to the wild type due to a block a
t stage I, i.e., a failure to form the polar division septum. In early stat
ionary phase, mutant cells accumulated intracellular and extracellular conc
entrations of citrate and isocitrate that,were at least 15-fold higher than
in wild-type cells. The growth and sporulation defects of the mutant could
be partially bypassed by deletion of the major citrate synthase gene (citZ
), by raising the pH of the medium, or by supplementation of the medium wit
h certain divalent cations, suggesting that abnormal accumulation of citrat
e affects survival of stationary-phase cells and sporulation by lowering ex
tracellular pH and chelating metal ions. While these genetic and environmen
tal alterations were not sufficient to allow the majority of the mutant cel
l population to pass the stage I block (lack of asymmetric septum formation
), introduction of the sof-1 mutant form of the Spo0A transcription factor,
when coupled with a reduction in citrate synthesis, restored sporulation g
ene expression and spore formation nearly to wild-type levels. Thus, the pr
imary factor inhibiting sporulation in a citC mutant is abnormally high acc
umulation of citrate, but relief of this metabolic defect is not by itself
sufficient to restore competence for sporulation.