mRNA Turnover

A major aspect of messenger RNA metabolism in eukaryotes is the events occurring after transcription, events that are necessary for messages to move from the nucleus to the cytosol where translation occurs (see here). In prokaryotes, by contrast, mRNAs are used in protein synthesis directly. In fact, a nascent mRNA serves as a template for translation while still in the process of being synthesized.

mRNA degradation - The major posttranscriptional event in the metabolism of prokaryotic mRNA is its own degradation, which in most cases is quite rapid. A few bacterial mRNAs, notably those encoding outer membrane proteins, are long-lived; however, most bacterial messages have half-lives of only 2 to 3 minutes. This short life span means that genes that are expressed must be transcribed continuously and that most mRNA molecules are translated only a few times. Although this might seem energetically wasteful, it is consistent with prokaryotic lifestyles, which necessitate rapid adaptation to environmental changes. There is a selective advantage to bacteria for expressing the genes for lactose utilization only when an inducer is present. By the same token, it would be wasteful for the cell to continue producing these proteins after lactose was exhausted. Rapid degradation of lac mRNA ensures that the energetically wasteful synthesis of these proteins will cease soon after the need for the proteins is gone.

Mechanism of degradation - Surprisingly little is known about the pathway of degradation. There are probably overlapping mechanisms, involving hydrolysis by nucleases and phosphorolysis by polynucleotide phosphorylase. It is known that degradation starts from the 5' end, which is important because translation also starts from the 5' end. If degradation were to start from the 3' end, a ribosome starting from a 5' end might never reach an intact 3' end. There is reason to think that mRNA degradation sometimes starts with the action of ribonuclease III, an enzyme specific for duplex RNA, which could cleave in stem-loop structures and create sites for exonucleolytic attack. RNase III is actually involved in the maturation of certain phage mRNAs as they undergo posttranscriptional processing, but this involvement is not known to occur with bacterial mRNAs.

An additional posttranscriptional process, called intron splicing, is almost exclusively confined to eukaryotes (see here).

See also: Posttranscriptional Processing of rRNA and tRNA, Processing of mRNA