Oxygen- and growth rate-dependent regulation of Escherichia coli fumarase (FumA, FumB, and FumC) activity

Escherichia coli contains three biochemically distinct fumarases which cata lyze the interconversion of fumarate to L-malate in the tricarboxylic acid cycle, Batch culture studies indicated that fumarase activities varied acco rding to carbon substrate and cell doubling time. Growth rate control of fu marase activities in the wild type and mutants was demonstrated in continuo us culture; FumA and FumC activities were induced four- to fivefold when th e cell growth rate (k) was lowered from 1.2/h to 0.24/h at 1 and 21% O-2, r espectively. There was a twofold induction of FumA and FumC activities when acetate was utilized instead of glucose as the sole carbon source. However , these fumarase activities were still shown to be under growth rate contro l. Thus, the activity of the fumarases is regulated by the cell growth rate and carbon source utilization independently, Further examination of FumA a nd FumC activities in a cya mutant suggested that growth rate control of Fu mA acid FumC activities is cyclic AMP dependent, Although the total fumaras e activity increased under aerobic conditions, the individual fumarase acti vities varied under different oxygen levels. While FumB activity was maxima l during anaerobic growth (k = 0.6/h), FumA was the major enzyme under anae robic cell growth, and the maximum activity was achieved when oxygen was el evated to 1 to 2%. Further increase in the oxygen level caused inactivation of FumA and FumB activities by the high oxidized state, but FumC activity increased simultaneously when the oxygen level aas higher than 4%. The same regulation of the activities of fumarases in response to different oxygen levels was also found in mutants. Therefore, synthesis of the three fumaras e enzymes is controlled in a hierarchical fashion depending on the environm ental oxygen that the cell encounters.