ACETATE UTILIZATION IS INHIBITED BY BENZOATE IN ALCALIGENES-EUTROPHUS- EVIDENCE FOR TRANSCRIPTIONAL CONTROL OF THE EXPRESSION OF ACOE CODING FOR ACETYL-COENZYME-A SYNTHETASE

During batch growth of Alcaligenes eutrophus on benzoate-acetate mixtu res, benzoate was the preferred substrate, with acetate consumption be ing delayed until the rate of benzoate consumption had diminished, Thi s effect was attributed to a transcriptional control of the synthesis of acetyl coenzyme A (acetyl-CoA) synthetase, an enzyme necessary for the entry of acetate into the central metabolic pathways, rather than to a biochemical modulation of the activity of this enzyme, Analysis o f a 2.4-kb mRNA transcript hybridizing with the A. eutrophus acoE gene confirmed this repression effect, In a benzoate-limited chemostat cul ture, derepression was observed, with no increase in the level of expr ession following an acetate pulse. Benzoate itself was not the signal triggering the repression of acetyl-CoA synthetase, This role was play ed by catechol, which transiently accumulated in the medium when high specific rates of benzoate consumption were reached. The lack of rapid inactivation of the functional acetyl-CoA synthetase after synthesis has been stopped enables A. eutrophus to retain the capacity to metabo lize acetate for prolonged periods while conserving minimal protein ex penditure.