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

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Abstract
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, pyrB553, pyrB730 and pyrB748, were cloned and their nucleotide sequences were determined along with that of the wild-type pyrB1 operon in order to locate the sites of the alterations in the catalytic chains. Misense mutation pyrB554 leads to replacement of Se-52 by phenylalanine and the inactive mutant enzyme has properties similar to those of wild-type aspartate transcarbamoylase. The amber mutation pyB730 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 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.