Genetic Organization and Structural Modelization of Aspartate Carbamoyltransferase from Psychrophilic and Barotolerant Bacteria
Ying XU*a, Vincent VILLERETb, Yuanfu ZHANGc, Ziyuan LIANGa, Mark VAN DE CASTEELEa, Christianne LEGRAINd, Jurgen RILLAERTSa, Jozef VAN BEEUMENb, and Nicolas GLANSDORFFa,d
a Erfelijkheidsleer en Microbiologie, Vrije Universiteit Brussel and Flanders Interuniversity Institute for Biotechnology, Ave. E. Gryson 1, B-1070 Brussels, Belgium
b Laboratorium voor Eiwitbiochimie en Eiwitengineering, Universiteit Gent, Ledeganckstraat 35, B-9000 Gent, Belgium
c Epidemiology Institute, Beijing, The Peoples Republic of China
d Research Institute of the CERIA, Ave. E. Gryson 1, B-1070 Brussels, Belgium
Aspartate carbamoyltransferase (ATCase) catalyzes the committed step of pyrimidine biosynthesis. E. coli ATCase has become a paradigm for the analysis of long-range interactions in allosteric enzymes; it is also the reference par excellence for comparative studies of ATCases from organisms adapted to the full range of temperatures compatible with life. We have cloned and sequenced the ATCase genes from two obligate psychophilic Vibrio-like strains isolated from the deep Atlantic (1) and analyzed the cognate protein sequences by structural modeling.
In both Vibrio strains and as in E. coli, the protein is encoded by an operon comprising a promoter-leader region probably regulated by attenuation and immediately followed by two adjacent genes, pyrB and pyrI, very similar to their E. coli equivalents (74.4 and 55.7% identical amino acid residues respectively) and 95% identical to each other. This similarity in sequence and in genetic organization suggests that Vibrio and E. coli ATCases have the same quaternary structure: two catalytic trimers of the pyrB-encoded subunit (c) held together by their association with three dimers of the pyrI-encoded regulatory subunit (r). The molecular mass of the enzyme supports this view.
As in E. coli the enzyme is allosteric, it displays homotropic interactions for aspartate and heterotropic interactions for CTP, which inhibits the enzyme. Contrary to E. coli ATCase however, Vibrio ATCase is not activated by ATP, nor synergistically inhibited by CTP and UTP. With respect to E. coli the temperature profile of Vibrio ATCase is shifted towards lower values, with a maximum at 30 to 35℃. Vibrio ATCase is considerably more thermolabile than its mesophilic or thermophilic homologues.
Modeling of the structure of the two Vibrio ATCases on the known 3D-structure
