AEROBIC REGULATION OF ISOCITRATE DEHYDROGENASE GENE (ICD) EXPRESSION IN ESCHERICHIA-COLI BY THE ARCA AND FNR GENE-PRODUCTS

Isocitrate dehydrogenase, the icd gene product, has been studied exten sively regarding the regulation of enzymatic activity and its relation ship to the metabolic flux between the tricarboxylic acid cycle and th e glyoxylate bypass. In this study, the transcriptional regulation of icd gene expression was monitored by using an icd-lacZ gene fusion and shown to vary over a 15-fold range in response to changes in oxygen a nd carbon availability. Anaerobic cell growth resulted in fivefold-low er icd-lacZ expression than during aerobic growth. This negative contr ol is mediated by the arcA and fnr gene products. When different carbo n compounds were used for cell growth, icd-lacZ expression varied thre efold. The results of continuous cell culture studies indicated that t his control may be due to variations in cell growth rate rather than t o catabolite repression. DNase I footprinting at the icd promoter reve aled a 42-bp ArcA-phosphate-protected region that overlaps the start s ite of icd transcription. Phosphorylation of ArcA considerably enhance d its binding to DNA, while ArcA phosphate exhibited an apparent disso ciation value of approximately 0.1 mu M. Based on these studies, ArcA appears to function as a classical repressor of transcription by bindi ng at a site overlapping the icd promoter during anaerobic cell growth conditions.