REGULATION OF MALATE-DEHYDROGENASE (MDH) GENE-EXPRESSION IN ESCHERICHIA-COLI IN RESPONSE TO OXYGEN, CARBON, AND HEME AVAILABILITY

Malate dehydrogenase catalyzes the interconversion of malate and oxaloacetate. It participates as a member of the tricarboxylic acid cycle and the branched noncyclic pathways under aerobic and anaerobic cell growth conditions, respectively, To investigate how the mdh gene is expressed under these different conditions, an mdh-lacZ operon fusion,vas constructed and analyzed in vivo. The mdh-lacZ fusion was expressed about twofold higher under aerobic conditions than under anaerobic cell growth conditions on most media tested, This anaerobic response is modulated by the ArcA protein, which functions as a repressor of mdh gene expression under both aerobic and anaerobic conditions, In contrast, mutations in the fnr gene did not affect mdh gene expression, Interestingly, cells grown anaerobically with glycerol and trimethylamine N-oxide or fumarate showed higher levels of mdh expression than did cells that were grown aerobically, Depending on the type of carbon compound used for cell growth, mdh expression varied by 11-fold and 5-fold under aerobic and anaerobic conditions, respectively. While mdh transcription was shown to be inversely proportional to the cell growth rate, cellular heme limitation stimulated a fivefold increase in mdh gene expression, The mdh gene appears to be highly regulated to adapt to changing conditions of aerobic and anaerobic cell growth with various types of carbon substrates.