DELETION OF THE BACILLUS-SUBTILIS ISOCITRATE DEHYDROGENASE GENE CAUSES A BLOCK AT STAGE-I OF SPORULATION

A Bacillus subtilis mutant with a deletion of citC, the gene encoding isocitrate dehydrogenase, the third enzyme of the tricarboxylic acid b ranch of the Krebs cycle, had a greatly reduced ability to sporulate, Analysis of expression of lacZ fusions to various sporulation gene pro moters revealed that in the citC mutant development is probably blocke d between stage 0 and stage II. That is, genes expressed very early in sporulation, under the direct control of the Spo0A transcription fact or, were induced normally in the citC mutant. However, genes expressed after asymmetric septation (stage II) in wild-type cells were not ind uced in the citC mutant. Analysis of cell morphology by thin-section e lectron microscopy and immunofluorescence microscopy showed that the m utant formed axial chromosomal filaments and accumulated rings of FtsZ protein at potential polar division sites but failed to form asymmetr ic division septa, indicating that sporulation is blocked at stage I. The growth and sporulation defects of the B. subtilis citC mutant were fully overcome by introduction and expression of the Escherichia coli icd gene, encoding an isocitrate dehydrogenase similar to the enzyme from B. subtilis.