The aspartate carbamoyltransferase (ATCase) genes of psychrophilic strain 2693 were cloned by complementation in and the enzyme was partly characterized. The genes constitute a operon homologous to the cognate structure in where and respectively encode the catalytic and the regulatory chains of ATCase. The strong sequence similarities noted between and ATCases include extensive conservation of residues involved in interactions between subunits, suggesting that the two enzymes have very similar tertiary and quaternary structures. ATCase is, however, not activated by ATP and not synergistically inhibited by CTP and UTP. It is also much more thermolabile than ATCase. With respect to and ATCases, ATCase presents marked differences in composition which could be related to its psychrophilic character. The results of these structural and functional comparisons indicate that 2693 ATCase is a suitable model for biochemical studies 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 transcription attenuator. These genetic and enzymic data strengthen the evolutionary relationship already noted between Vibrionaceae and Enterobacteriaceae.


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