Location of amino acid alterations in mutants of aspartate transcarbamoylase: Structural aspects of interallelic complementation.

Recent genetic studies of the pyrB locus of Escherichia coli resulted in the characterization of 29 mutant strains harboring defects in the structural gene that encodes the catalytic chains of aspartate transcarbamoylase (carbamoylphosphate: L-aspartate carbamoyltransferase, EC 2.1.3.2). Three alleles, pyrB554, pyrB730, and pyrB748, have been cloned, and their nucleotide sequences have been determined along with that of the wild-type pyrBI operon in order to locate the sites of the alterations in the catalytic chains. Missense mutation pyrB554 leads to replacement of serine-52 by phenylalanine, and the inactive mutant enzyme has properties similar to those of wild-type aspartate transcarbamoylase. The amber mutation pyrB730 results in unstable truncated polypeptide chains 27 amino acids shorter than wild-type chains. Deletion mutation pyrB748 causes the removal of 181 amino acids. Combining these results with knowledge of the crystallographic structure of the wild-type enzyme provides a basis for tentative structural mechanisms for the observed complementation behavior of the mutant proteins.