Isolation and Preliminary Characterization of Single Amino Acid Substitution Mutants of Aspartate Carbamoyltransferase

In order to isolate functional Escherichia coli aspartate carbamoyltransferase (carbamoylphosphate:L-aspartate carbamoyltransferase, EC 2.1.3.2) with single amino acid replacements, a series of pyrB nonsense mutants has been isolated. These nonsense mutants were induced by 2-aminopurine mutagenesis and selected by a combination of antibiotic treatments, direct enzyme assays, and suppressibility tests. Suppression of the pyrB nonsense mutation with various suppressors, which insert different amino acids, has resulted in the formation of a series of mutant aspartate carbamoyltransferases, each differing in one amino acid from the wild-type enzyme. After partial purification, kinetic studies revealed that some of the mutant enzymes had altered homotropic and heterotropic interactions. The mutants that had a tyrosine insert showed the most pronounced changes, followed by those with a serine insert. The mutants having a glutamine insert, however, were indistinguishable from the wild-type enzyme, supporting the conclusion that, because of the specificity of the mutagen, the glutamine insert had regenerated the wild-type enzyme.