The transcriptional regulation of the gua Operon of Escherichia coli

Transcriptional regulation of the gua operon of Escherichia coli has been studied. Studies with the messenger RNA encoding IMP dehydrogenase and GMP synthase (quaBA transcript) using the technique of reverse transcriptase primer extension have mapped the location of the qua promoter. Sequence determinants in the region of the qua promoter indicate potential control mechanisms for qua expression such as stringent and growth-rate-dependent regulation. To investigate the possibility of regulation by these mechanisms, qua-lac fusions were constructed in vitro and were used to demonstrate that qua expression is controlled by both these mechanisms in vivo, and that growth-rate control is independent of guanine-mediated repression. A qua-lac fusion in which the GC-rich discriminator was mutated by insertion of AT-rich DNA was used to demonstrate the importance of this region in control. Both stringent and growth-rate-dependent controls were abolished in the mutant. To demonstrate that control is a direct effect of ppGpp on qua expression, transcription in vitro was carried out in the presence or absence of ppGpp. Expression from the wildtype qua promoter was inhibited by ppGpp whilst expression from the mutant was unaffected, confirming ppGpp-mediated control of qua expression in vivo and the importance of the discriminator in this control.