DNA sequence of the Escherichia coli gene, gnd, for 6-phosphogluconate dehydrogenase

Expression of gnd of Escherichia coli, which encodes 6-phosphogluconate dehydrogenase, an enzyme of the hexose monophosphate shunt, is subject to growth rate-dependent regulation and is gene dosage-dependent: the level of the enzyme increases in direct proportion to the cellular growth rate at both low and high gene copy numbers. We have determined the nucleotide sequence of gnd and flanking control regions, the 5'-end of in vivo gnd mRNA, and the start codon of the structural gene. Analysis of the sequence indicated that: (i) the gnd promoter is typical of other E. coli promoters and the structural gene is followed by a rho-independent transcription termination signal; (ii) the 56-nucleotide leader of gnd mRNA does not contain a rho-independent transcription termination signal, so growth rate-dependent regulation of 6-phosphogluconate dehydrogenase level is not carried out by an attenuation mechanism analogous to the one that controls expression of the E. coli ampC gene; (iii) the codon composition of the structural gene resembles that of other highly expressed E. coli genes and thus is not responsible for the regulation either; (iv) the structural gene is preceded at an optimal distance by a strong Shine-Dalgarno (SD) sequence, AGGAG ; (v) the leader region of the mRNA contains regions of dyad symmetry that have the potential to sequester the SD sequence and the start codon. This latter feature of the gene suggests that growth rate-dependent regulation may involve regulation of translation initiation frequency.