Abstract

The serA gene in Escherichia coli encodes 3-phosphoglycerate dehydrogenase, which is the first enzyme in the serine biosynthesis pathway. Previous studies have determined two different transcription initiation sites for the serA gene, corresponding to the promoters of serA P1 and P2, and suggested that P1 and P2 were regulated by leucine responsive regulatory protein (Lrp) by different mechanisms. However, very little is known about how the two promoters function in different cell growth conditions, and whether other regulator(s) are involved in expression and regulation of serA . In this study, I confirm that Lrp activates P1 but represses P2 in glucose minimal medium by studying native and mutant promoters as serA::IacZ operon fusions. Transcription can take place from both P1 and P2 promoters when Lrp is absent. Furthermore, my study demonstrates that the cyclic AMP (cAMP) receptor protein-cAMP (cAMP-CRP) complex may positively regulate serA by activating P2 and is mainly functional in the absence of Lrp.