The E. coli carbamoylphosphate synthetase operon: a paradigm of complex bacterial regulation

E. coli has, in contrast to most other microorganisms, only one single carbamoylphosphate synthetase (CPSase). This particular situation, creating a direct link between the biosynthesis of arginine and of the pyrimidines, is reflected in the complex regulation of enzyme activity and of enzyme production. The carAB operon is transcribed from two promoters; modulation of their activity integrates signals from three distinct pathways, arginine, pyrimidines and purines and requires at least five regulatory proteins, ArgR (arginine repressor), PepA (aminopeptidase A), IHF (integration Host Factor), PyrH (UMP-kinase) and PurR (purine repressor). The downstream P2 promoter is mainly regulated by arginine, while the upstream P1 is regulated by pyrimidine and purine nucleotides. Moreover, P1 expression is subject to UTP-dependent reiterative transcription and to stringent control (ppGpp). Interestingly, nearly all these regulatory proteins are multifunctional: PepA and PyrH combine their role in transcriptional regulation with a catalytic activity and, even more unexpected, ArgR and PepA play a joint architectural role in the site-specific DNA resolution reaction of ColE1 multimers. We focus on the analysis of protein-DNA and protein-protein interactions involved in pyrimidine and purine-specific regulation of P1 activity, on the importance of DNA conformability in the elaboration of the intricate regulatory nucleoprotein complexes and on the mutual influence of P1 and P2 promoter activity.