MOLECULAR PHYSIOLOGY OF CARBAMOYLATION HINDER EXTREME CONDITIONS - WHAT CAN WE LEARN FROM EXTREME THERMOPHILIC MICROORGANISMS

The importance of protein-protein interactions in the physiology of ex treme thermophiles was investigated by analyzing the enzymes involved in biosynthetic carbamoylation in Thermus ZO5 and by comparing the res ults obtained with already available or as yet unpublished information concerning other thermophilic eu-and archaebacteria such as Thermotog a, Sulfolobus, and Pyrococcus. Salient observations were that (i) the highly thermolabile and reactive carbamoylphosphate molecule appears t o be protected from thermodegradation by channelling towards the synth esis of citrulline and carbamoylaspartate, respectively precursors of arginine and the pyrimidines; (ii) Thermus ornithine carbamoyltransfer ase is clearly a thermophilic enzyme, intrinsically thermostable and s howing a biphasic Arrhenius plot, whereas aspartate carbamoyltransfera se is inherently unstable and is stabilized by its association with di hydroorotase, another enzyme encoded by the Thermus pyrimidine operon. Possible implications of these results are discussed. (C) 1997 Elsevi er Science Inc.