The endoribonuclease RNase E plays an important role in RNA processing and degradation in Escherichia coli. The construction of an E. coli strain in which the cellular concentration of RNase E can be precisely controlled has made it possible to examine and quantify the effect of RNase E scarcity on RNA decay, gene regulation and cell growth. These studies show that RNase E participates in a step in the degradation of its RNA substrates that is partially or fully rate-determining. Our data also indicate that E. coli growth requires a cellular RNase E concentration at least 10-20% of normal and that the feedback mechanism that limits overproduction of RNase E is also able to increase its synthesis when its concentration drops below normal. The magnitude of the in-crease in RNA longevity under conditions of RNase E scarcity may be limited by an alternative pathway for RNA degradation. Additional experiments show that RNase E is a stable protein in E. coli. No other E. coli gene product, when either mutated or cloned on a multicopy plasmid, seems to be capable of compensating for an inadequate supply of this essential protein.