An important characteristic of bacterial mRNA is that its lifetime is short compared to other types of bacterial RNA molecules. The half-life of a typical bacterial mRNA molecule is a few minutes. This feature, which may seem terribly wasteful, has an important regulatory function. A cell can turn off the synthesis of a protein that is no longer needed by turning off synthesis of the mRNA that encodes the protein. Soon after, none of that particular mRNA will remain and synthesis of the protein will cease. Of course, this regulation also means that in order to maintain synthesis of a particular protein, the mRNA molecules encoding these proteins must be synthesized continuously. Continuous mRNA synthesis is a small payment by the cell for the ability to regulate the synthesis of specific proteins. This means that in the overall metabolism of a cell, much less ATP is consumed than would be used if synthesis of proteins encoded in the mRNA continued long after the proteins were no longer needed.
    The short lifetime of bacterial mRNA is one criterion used to identify mRNA in bacteria. A common experimental technique to determine whether a particular RNA molecule or class of RNA molecules is mRNA is the pulse-chase experiment. RNA is labeled briefly by growing bacteria in the presence of a radioactive precursor such as [3H]uridine. Then the bacteria are switched to a medium containing no [3H]uridine and a high concentration of nonradioactive urdine and samples are removed at specific times for analyses. The RNA is isolated and different species are separated by gel electrophoresis or centrifugation and detected by their radioactivity. A stable radioactive RNA molecule will be present through many generations, whereas a radioactive mRNA molecule will decrease with a half-life of two to three minutes. One difficulty with this technique is that bacteria contain some long-lived mRNA molecules and these would be misclassified. A better criterion for identifying a molecule as mRNA would be to isolate it and determine in an in vitro protein-synthesizing system whether the particular RNA can direct protein synthesis.