Intracellular mRNA content depends on the rate at which mRNA is degraded as well as the rate at which it is synthesized. The typical E.coli mRNA molecule has a half-life of about two to three minuteS. However, some mRNA half-lives are about 30 minutes while others are as short as a few seconds. Bacteria derive an important advantage from rapid mRNA degradation. If mRNA molecules were stable, newly synthesized mRNA molecules would have to compete with preexisting mRNA molecules for the protein synthetic machinery. Such competition would limit the cell's ability to synthesize proteins that are needed to respond to physiological changes. Rapid mRNA degradation frees the protein synthetic machinery to translate the newly formed mRNA molecules, which are formed in response to the cell's changing physiological requirements.
    Bacteria use several enzymes to degrade mRNA. These enzymes can be divided into two major classes, polynucleotide phosphorylases and ribonucleases (RNases). A polynucleotide phosphorylase de-grades RNA by adding phosphate groups to phosophodiester bonds to form nucleoside diphosphates. Phosphorolytic cleavage begins at the 3’-end and continues sequentially in a 3’→5’ direction. RNases cleave phosphodiester bonds to produce RNA fragments, while exoribonucleases cut phosphodiester bonds in a sequential fashion starting at the 3’-end and moving 3’→5’ to produce nucleoside monophosphates. To date, no bacterial degradation enzyme has been found that catalyzes sequential RNA cleavage in a 5’→ 3’ direction. In contrast, bacteria do have exonucleases that remove nucleotides from DNA in a 5’→ 3’ direction. Although the study of mRNA degradation is still in its infancy, the involvement of certain enzymes in this process is well documented. We now examine a few of these enzymes and explore their role in mRNA degradation.