A Model for the Common Control of Enzymes of Ethanolamine Catabolism in Escherichia coli

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Abstract
By varying the composition of the growth medium and the genotype of the bacterial strain, 5 isoenzymes of CoA-dependent aldehyde dehydrogenase (EC 1.2.1.10) could be detected in E. coli. Two isoenzymes (A, MW wt 520,000; and B, MW 370,000) were produced only in the presence of ethanolamine and vitamin (or coenzyme) B12 (inducible isoenzymes). The other 3 isoenzymes (C, MW 900,000; D, MW 120,000; and E, MW 720,000) were produced only in the absence of ethanolamine and vitamin B12 (repressible isoenzymes). Partial purification and characterization of these isoenzymes revealed strong similarities, with respect to pH optima and substrate affinities, between isoenzymes within the 2 classes, but significant differences between the 2 classes. Mutant studies demonstrated that the relationships between the isoenzymes and between CoA-dependent aldehyde dehydrogenase and ethanolamine ammonia-lyase are both structural and regulatory in nature, and a 2-operon model is proposed to account for the common control of the enzymes of ethanolamine catabolism in E. coli.