Genes Encoding Thermophilic Aspartate Carbamoyltransferases ofThermus AquaticusZ05 andThermotoga MaritimaMSB8: Modes of Expression inE. Coliand Properties of Their Products

Aspartate carbamoyltransferase genes from the extreme thermophilic eubacteria Ta. maritima and T. aquaticus were cloned by complementation in E. coli.Sequencing of the Ta. maritima pyrB gene, the aberrant behaviour of the enzyme product in E. coli, and comparison of the derived amino acid sequence with mesophilic ATCases suggest that the gene was disrupted in the process of cloning and that Thermotoga ATCase belongs to an unusual class of aspartate carbamoyltransferases.Analysis of the proximal part of the T. aquaticus pyr operon and characterization of the ATCase gene products formed in E. coli and in the original host led to the proposal that the T. aquaticus aspartate carbamoyltransferase and dihydroorotase enzymes associate to form a stable multienzyme complex, regulated by UTP.Some indications of how the thermophilic ATCase genes could be expressed in E. coli were also obtained.