Messenger RNA Turnover in Cell-Free Extracts from Higher Eukaryotes

Two groups of observations support the notion that mRNA turnover influences gene expression in virtually all cells (1 –3 ). (1) The steady-state levels of many mRNAs are determined more by their half-lives than by their gene transcription rates. In other words, mRNA levels often fluctuate without any measurable change in transcription. (2) The half-lives of some mRNAs change when the environment of the cell, its replication cycle status, or its stage of differentiation changes. The sequences and structures that determine the half-lives of many mRNAs have been mapped in great detail. In contrast, little is known about the RNases that specifically degrade mRNAs (mRNases) and the regulatory factors that influence mRNA stability. How many mRNases are expressed in each cell? Do different cells express different mRNases with different specificities? How do these enzymes function? Are they endonucleases or exonucleases, and where do they first begin to attack the mRNA molecule? What are the major pathways of mRNA decay, and what sorts of degradation intermediates are generated as mRNAs are destroyed? How does translation affect mRNA turnover? What trans -acting factors regulate mRNA turnover? How do they function—by binding to the mRNA molecules they affect, by up- or down-regulating mRNase activity, or by indirect mechanisms?