| Abstrakt: |
The aspartate transcarbamoylases (ATCase, EC 2.1.3.2) of Escherichia coli and Serratia marcescenshave similar dodecameric enzyme structures (2(c3):3(r2] but differ in both regulatory and catalytic characteristics. The catalytic cistrons (pyrB) of the ATCases from E. coliand S. marcescensencode polypeptides of 311 and 306 amino acids, respectively; there is a 76% identity between the DNA sequences and an overall amino acid homology of 88% (38 differences). The regulatory cistrons (pyrI) of these ATCases encode polypeptides of 153 and 154 amino acids, respectively, and there is a 75% identity between the DNA sequences and an overall amino acid homology of 77% (36 differences). In both species, the two genes are arranged as a bicistronic operon, with pyrBpromoter proximal. A comparison of the deduced amino acid sequences reveals that the active site and the allosteric binding sites, as well as most of the intrasubunit interactions and intersubunit associations, are conserved in the E. coliand the S. marcescensenzymes; however, there are specific differences which undoubtedly contribute to the catalytic and regulatory differences between the enzymes of the two species. These differences include residues that have been implicated in the T-R transition, c1:r1 interface interactions, and the CTP binding site. A hybrid ATCase assembled in vivowith catalytic subunits from E. coliand regulatory subunits from S. marcescenshas a 6 mMrequirement for aspartate at half-maximal saturation, similar to the 5.5 mMaspartate requirement of the native E. coliholoenzyme at half-maximal saturation. However, the heterotropic response of this hybrid enzyme is characteristic of the heterotropic response of the native S. marcescensholoenzyme: ATP activation and CTP activation. Activation by both allosteric effectors indicates that the heterotropic response of this hybrid holoenzyme (Cec: Rsm) is determined by the associated S. marcescensregulatory subunits. |
