تسجيل الدخول

تسجيل

RNA Processing:- Cellular mRNAs Are Degraded at Different Rates

المؤلف:  David L. Nelson، Michael M. Cox

المصدر:  Lehninger Principles of Biochemistry

الجزء والصفحة:  1020

2026-07-25

29

RNA Processing:- Cellular mRNAs Are Degraded at Different Rates

The expression of genes is regulated at many levels. A crucial factor governing a gene’s expression is the cellular concentration of its associated mRNA. The con centration of any molecule depends on two factors: its rate of synthesis and its rate of degradation. When syn thesis and degradation of an mRNA are balanced, the concentration of the mRNA remains in a steady state. A change in either rate will lead to net accumulation or depletion of the mRNA. Degradative pathways ensure that mRNAs do not build up in the cell and direct the synthesis of unnecessary proteins. The rates of degradation vary greatly for mRNAs from different eukaryotic genes. For a gene product that is needed only briefly, the half-life of its mRNA may be only minutes or even seconds. Gene products needed constantly by the cell may have mRNAs that are stable over many cell generations. The average half-life of a vertebrate cell mRNA is about 3 hours, with the pool of each type of mRNA turning over about ten times per cell generation. The half-life of bacterial mRNAs is much shorter—only about 1.5 min—perhaps because of regulatory requirements. Messenger RNA is degraded by ribonucleases present in all cells. In E. coli, the process begins with one or a few cuts by an endoribonuclease, followed by 3→5 degradation by exoribonucleases. In lower eukaryotes, the major pathway involves first shortening the poly(A) tail, then decapping the 5 end and degrading the mRNA in the 5n3 direction. A 3→5 degradative pathway also exists and may be the major path in higher eukaryotes. All eukaryotes have a complex of up to ten conserved 3n5 exoribonucleases, called the exosome, which is involved in the processing of the 3end of rRNAs and tRNAs as well as the degradation of mRNAs. A hairpin structure in bacterial mRNAs with a

Ρ-independent terminator  confers stability against degradation. Similar hairpin structures can make some parts of a primary transcript more stable, leading to nonuniform degradation of transcripts. In eukaryotic cells, both the 3 poly(A) tail and the 5 cap are important to the stability of many mRNAs.