The gnd gene of Escherichia coli encodes 6-phosphogluconate dehydrogenase (6PGD), an enzyme of the hexose monophosphate shunt pathway of central intermediary metabolism. The levels of 6PGD are regulated by the cellular growth rate, increasing fourfold with increasing growth rate. Although this variation quantitatively resembles that observed for rRNA and r-proteins, results of our unpublished experiments on the effects of nutritional shifts and amino acid starvation on the rate of 6PGD synthesis demonstrated that the mechanism for growth rate control of 6PGD synthesis is distinct from that which regulates synthesis of rRNA and r-protein and is common to the regulation of many other proteins. The objective of the present proposal is to elucidate the mechanisms underlying the growth rate-dependent control of 6PGD synthesis. Several models for regulation of gnd are presented and used as a conceptual base for the design of experiments. Strains carrying gnd-lac operon and gene fusions will be prepared by the methods of Casadaban using our previously isolated gnd::Mucts mutants. The fusions will be used to determine whether growth rate-dependent control of 6PGD synthesis is transcriptional or post-transcriptional and for the isolation of mutants with mutations in gnd or unlinked to gnd which alter growth rate control. The DNA sequence of the gnd control region will be determined for wild type and mutant genes and for gnd of Salmonella typhimurium, which shows an altered growth rate response. S30 extracts will be prepared from wild type cells grown at different rates and from mutants, and used in DNA-dependent in vitro systems to identify and characterize the regulators of growth rate dependent control of 6PGD synthesis.
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