ASPARTATE CARBAMOYLTRANSFERASE FROM A PSYCHROPHILIC DEEP-SEA BACTERIUM, VIBRIO STRAIN-2693 - PROPERTIES OF THE ENZYME, GENETIC ORGANIZATIONAND SYNTHESIS IN ESCHERICHIA-COLI

The aspartate carbamoyltransferase (ATCase) genes of psychrophilic Vib rio strain 2693 were cloned by complementation in Escherichia coli and the enzyme was partly characterized. The genes constitute a pyrBI ope ron homologous to the cognate structure in E. coli where pyrB and pyrI respectively encode the catalytic and the regulatory chains of ATCase . The strong sequence similarities noted between Vibrio and E. coli AT Cases include extensive conservation of residues involved in interacti ons between subunits, suggesting that the two enzymes have very simila r tertiary and quaternary structures. Vibrio ATCase is, however, not a ctivated by ATP and not synergistically inhibited by CTP and UTP. It i s also much more thermolabile than E. coil ATCase. With respect to Pyr ococcus abyssi and E. coil ATCases, Vibrio ATCase presents marked diff erences in composition which could be related to its psychrophilic cha racter. The results of these structural and functional comparisons ind icate that Vibrio 2693 ATCase is a suitable model for biochemical stud ies on structure-function relationships in a 'cold' allosteric enzyme. the operon is expressed from a promoter which is immediately followed by a pyrimidine-rich leader ORF terminating within a putative transcr iption attenuator. These genetic and enzymic data strengthen the evolu tionary relationship already noted between Vibrionaceae and Enterobact eriaceae.