A mutation in the catalytic cistron of aspartate carbamoyltransferase affecting catalysis, regulatory response and holoenzyme assembly

We describe here a mutation in the gene encoding the catalytic subunit of aspartate carbamoyltransferase (ATCase, pyr B) which produces an enzyme retaining catalytic activity as holoenzyme (2C 3 :3R 2 ) and catalytic trimer (C 3 ) but which shows neither cooperative substrate kinetics nor nucleotide effector response. Furthermore, the holoenzyme assembly seems quite fragile in that the enzymatic activity is recovered only partially as holoenzyme following growth of Escherichia coli in the presence of zinc. In contrast, the enzyme from wild-type E. coli strain E63 is recovered almost entirely in its dodecameric form in identical conditions. The gene encoding the mutant pyrB was isolated from a λdargI + pyrB + transducing phage (obtained from N. Glansdorff). This mutation differs from other previously reported mutations affecting the catalytic subunit 1,2 in that significant catalytic activity is retained but homotropic and heterotropic communication is lost 3 .