Controls ability to utilize lactose, galactose, arabinose, maltose, tryptophan, D-serine, and histidine. EnzIIA Glu becomes phosphorylated as glucose supply runs out. System is very important. The regulation of enzyme levels, etc. must optimize the cell metabolism in ways that are still not understood completely. Mutant discovered in the 1950's could not reduce the rate of rRNA and tRNA synthesis.
Mutants show lags of hours when down-shifted nutritionally.
Wild-type adapts often in minutes. These compounds can be synthesized in two ways. Depletion of carbon or energy source can also trigger the stringent response, which serves to conserve ATP by reducing protein synthesis. What the SpoT protein might sense in the cell is not known. In any event, RNA polymerase pausing could be modified and interference with both initiation and elongation affected.