Biochemical and genetic divergence of aspartate- and ornithine-carbamoyltransferases of Escherichia coli

Abstract

The aspartate- and ornithine- carbamoyl-transferase enzymes of Escherichia coli are characterized by physical, chemical, and genetic similarities which suggest that these enzymes have evolved from a common evolutionary ancestor. Unlike aspartate carbamoyltransferase, ornithine carbamoyltransferase has not been extensively characterized on the primary, secondary, and tertiary level. In addition, the DNA sequence of the gene encoding the catalytic polypeptide of aspartate carbamoyltransferase, pyrB, is known. Reported herein is the DNA sequence of the structural gene for argI, which encodes the OTCase polypeptide, and the amino acid sequence of the OTCase polypeptide as deduced from the argI sequence The amino acid sequence of the OTCase and ATCase polypeptides were compared to allow inferences to be made concerning the divergence of these enzymes. These sequences were aligned to maximize sequence homology; regions lacking sequence homology were aligned on the basis of secondary structural probabilities using the procedure of Chou and Fasman. These polypeptides were found to possess about 25% amino acid homology. This alignment also predicts the secondary structure of the OTCase polypeptide based upon the known secondary structure of the ATCase polypeptide. In addition, several prominent regions of length polymorphism, all occurring on the enzyme surface are proposed for these enzymes. The argI and pyrB genes were also aligned in order to examine their evolutionary relationship. Many insertions and/or deletions were invoked to align these DNA sequences. Many of the insertions appear to have evolved by a slipped mispairing mechanism which allowed the duplication of tandem or near-tandem DNA sequences. These alterations often placed homologous DNA sequences into different reading frames, and it is proposed that such frameshift mechanisms were commonly used in the divergence of argI and pyrB.