Different control circuits for growth rate-dependent regulation of 6-phosphogluconate dehydrogenase and protein components of the translational machinery in Escherichia coli.

Previous studies showed that the level of 6-phosphogluconate (6PG) dehydrogenase increases about fourfold with increasing growth rate when the growth rate is varied by varying the carbon source. When the growth rate was reduced by anaerobic growth or by using mutations to divert metabolism to less efficient pathways, the level of 6PG dehydrogenase was the same as in a wild-type strain growing aerobically on other carbon sources that yielded the same growth rate. Thus, expression of gnd, which encodes 6PG dehydrogenase, is regulated by the cellular growth rate and not by specific nutrients in the medium. Growth rate-dependent regulation was independent of temperature. After a nutritional shift-up, 6PG dehydrogenase and total protein did not attain the postshift rate of accumulation for 30 min, whereas RNA accumulation increased immediately. The kinetics of accumulation of 6PG dehydrogenase and RNA were coincident after a nutritional shift-down. Partial amino acid starvation of a strain that controls RNA synthesis stringently (rel+) had no effect on the differential rate of accumulation of the enzyme. The level of 6PG dehydrogenase in cells harboring a gnd+ multicopy plasmid was in approximate proportion to gene dosage and somewhat higher at faster growth rates. Growth rate control of chromosomal gnd was normal in strains carrying multiple copies of the promoter-proximal and promoter-distal portions of gnd. These results show that gnd is not part of the same regulatory network as components of the translational apparatus since gnd shows a delayed response to a nutritional shift-up, is not autoregulated, and is not subject to stringent control. Models to account for growth rate-dependent regulation of gnd are discussed.