Publisher Summary The many demonstrations that the Escherichia coli (E. coli ) rne gene product (RNase E) is involved in messenger RNA (mRNA) decay have given real impetus to the study of this unusual protein's properties and role. The recent attention given to the polyadenylylation of bacterial mRNAs and the discovery that polyadenylylation plays a role in the turnover of an E. coli plasmid-specific RNA show that bacterial mRNA decay has similarities to the eukaryotic process. Functional decay of a messenger refers to the inactivation of a messenger's template activity, whereas chemical decay refers to the degradation of the mRNA itself. Functional decay, as a process distinct from the cleavages that initiate degradation, has been demonstrated for only a few mRNAs. Specific messengers have half-lives that range from about 0.5 to 20 minutes. The rates of decay of many messengers are actively regulated, thus, affecting the yields of the proteins synthesized. Unique to bacteria, portions of polycistronic mRNAs may decay at different rates, independent of the size of the segment or its position in the transcript. Overall, the decay of mRNA in bacterial cells is a major metabolic function. The turnover of mRNA constitutes about half of the cells' RNA synthesis; the actual amount is related to their growth rate.