Critical interchain and intrachain contacts involved inthe transmission of allosteric effects in Escherichia coli aspartate carbamoyltransferase (ATCase) have been identified. Adenosine triphosphate (ATP) and cytidine triphosphate (CTP) have been shown to exert their effects by different pathways even though they bind to the same regulatory site. Progress has been made in understanding substrate binding. Critical residues and domains involved in the allosteric behaviour of Pseudomonas aeruginosa catabolic ornithine carbamoyltransferase (OTCase) have been identified. The enzyme (wild type and regulatory mutant form) has been crystallized and shown to be a dodecamer. An in vivo gene fusion technique has been devised to study directed enzyme evolution. The pathway of arginine biosynthesis in several thermophilic eubacteria and archaebacteria has been clarified. OTCases and ATCases have been purified and partially characterized from Thermus aquaticus, Thermotoga maritima, Sulfolobus solfataricus and Pyrococcus furiosus. Several instances of extreme heat stability (in particular in the presence of substrate and/or products) have been brought to light. The genes for Thermus aquaticus and Thermotoga maritima ATCases have been cloned.