EFFECT OF CELL-GROWTH RATE ON EXPRESSION OF THE ANAEROBIC RESPIRATORYPATHWAY OPERONS FRDABCD, DMSABC, AND NARGHJI OF ESCHERICHIA-COLI

The fumarate reductase (frdABCD), dimethyl sulfoxide (DMSO)-trimethyla mine-N-oxide (TMAO) reductase (dmsABC), and nitrate reductase (narGHJI ) operons in Escherichia coil encode enzymes Involved in anaerobic res piration to the electron accepters fumarate, DMSO or TMAO, and nitrate , respectively. They are regulated in response to anaerobiosis and nit rate availability. To determine how each operon is regulated in respon se to changes in cell growth rate and in oxygen availability, expressi on of fLdA-lacZ dmsA-lacZ, and narG-lacZ fusion genes was examined dur ing continuous culture. After a change in the cell growth rate, each a naerobic electron transport pathway operon fusion responded somewhat d ifferently. Whereas frdA-lacZ expression increased by fivefold as the growth rate decreased from 0.60 to 0.12/hour during aerobic growth, li ttle change was seen under anaerobic conditions. In contrast, growth r ate-dependent expression of narG-lacZ expression occurred under anaero bic conditions but not under aerobic conditions. Finally, dmsA-lacZ ex pression did not vary greatly for any of the growth rates tested. When cells were shifted from aerobic to anaerobic growth conditions, expre ssion of each fusion increased at a moderate rate and peaked or ''over shot'' before reaching a new equilibrium value. This ''overshoot'' phe nomenon was independent of the fnr gene product, which functions as a transcriptional activator of each respiratory operon during anaerobic conditions. In contrast to the moderate rate of anaerobic induction se en for narG-lacZ expression, the addition of nitrate caused a rapid in duction response. The cell appears to have many ways to adjust cell re spiration in response to changes in cell growth conditions;