We studied the interaction of the tricarboxylic acid cycle enzymes citrate synthase, isocitrate dehydrogenase and malate dehydrogenase in the bacteria Bacillus subtilis and Escherichia coli in vivo. In B. subtilis, the genes encoding citrate synthase, isocitrate dehydrogenase and malate dehydrogenase form an operon (citZ-icd-mdh) and predominantly are co-transcribed from a single promoter. In E. coli the corresponding genes gltA, icd and mdh do not form a transcription unit, are scattered around the chromosome and are expressed from different promoters. We found that co-transcription of genes and subsequent co-translation of the corresponding mRNAs promotes the formation of protein complexes and give support for the previous findings that in B. subtilis citrate synthase, isocitrate dehydrogenase and malate dehydrogenase form an enzyme complex (metabolon).