Essential site for growth rate-dependent regulation within the Escherichia coli gnd structural gene

Expression of gnd of Escherichia coli, which encodes the hexose monophosphate shunt enzyme, 6-phosphogluconate dehydrogenase (6PGD; EC 1.1.1.44), is subject to growth rate-dependent regulation: the level of the enzyme is directly proportional to growth rate under a variety of growth conditions. Previous results obtained with strains carrying transcriptional fusions of gnd to the structural genes of the lactose operon suggested that the growth rate-dependent regulation of gnd expression is at the post-transcriptional level. To characterize the regulation further, we prepared with phage MudII a set of eight independent gnd-lac gene (protein) fusions. We showed through genetic analysis and DNA sequencing that each fusion joint was located within the 6PGD-coding sequence between the first and second base pair of a codon, the reading frame required for production of a hybrid 6PGD-beta-galactosidase. Strains harboring the gnd-lac fusion plasmids produced proteins whose mobility in a NaDodSO4/polyacrylamide gel agreed with the molecular weights predicted from the DNA sequence for the respective hybrid proteins. The level of beta-galactosidase was high and relatively growth rate-independent in the fusion whose fusion joint was at codon 48. The level of beta-galactosidase in the other seven fusion strains whose fusion joints were located further downstream in the 6PGD-coding sequence showed the same dependence on growth rate as 6PGD in a normal strain. beta-Galactosidase levels were not affected by the presence of a gnd+ gene in trans to any of the fusions. The results suggest that all sites necessary for growth rate-dependent regulation of 6PGD level lie in gnd upstream from codon 118 and that an essential site of negative control lies within the coding sequence, between codons 48 and 118.