MRNA, degradation processes

The other system controlling cholesterol biosynthesis involves both the cytosolic HMG-CoA synthase and the ER enzyme HMG-CoA reductase and is based on the cellular levels of respective mRNAs. Increasing free cholesterol decreases both enzyme activities by decreasing the levels of their mRNAs and increasing enzyme degradation processes. The half-life of HMG-CoA reductase may be as short as 1.7 h. Cellular uptake of LDL maintains cholesterol biosynthesis at a relatively low level, and this is achieved through an LDL degradation product-free cholesterol. Some authorities have maintained that hydroxylated... 

RNAi, first described in plants and termed cosuppression (reviewed in [21]), is a process in which double-stranded RNA induces homology-dependent degradation of mRNA [22-24]. RNAi is a process involving small interfering double-stranded... 

RNA Interference (RNAl) Is a process of post-transcriptlonal gene silencing mediated by short double-stranded RNA molecules called siRNA (small Interfering RNAs). In mammalian cells, transfection of 21-22 nucleotide siRNAs leads to degradation of mRNA molecules that contain the same sequence as the siRNA. In the following experiment, siRNA and knockout mice are used to Investigate two related cell surface proteins designated p24 and p25 that are suspected to be cellular receptors for the uptake of a newly Isolated virus. 

Because the human genome contains long introns, only 5 percent of the nucleotides that are polymerized by RNA polymerase II during transcription are retained in the mature, processed mRNA. The Introns that are spliced out and the region downstream from the cleavage and polyadenylatlon site are degraded by nuclear exonucleases that hydrolyze one base at a time from either the 5 or 3 end of an RNA strand. 

As mentioned earlier, the 2, 5 -phosphodlester bond in excised Introns is hydrolyzed by a debranching enzyme, yielding a linear molecule with unprotected ends that can be attacked by exonucleases (see Figure 12-9). The predominant nuclear decay pathway is 3 —>5 hydrolysis by 11 exonucleases that associate with one another in a large protein complex called the exosome. Other proteins in the complex Include RNA hell-cases that disrupt base pairing and RNA-proteln interactions that would otherwise Impede the exonucleases. Exosomes also function in the cytoplasm as discussed later. In addition the exosome appears to degrade pre-mRNAs that have not been properly spliced or polyadenylated. It is not yet clear how the exosome recognizes improperly processed pre-mRNAs. 

EXAMPLE 9.43 In 2006, a Nobel Prize was awarded for the discovery of RNA interference (RNAi). This is an exquisitely specific and sensitive process in which double-stranded RNA (dsRNA) guides distinct nucleases to degrade only mRNA that shares its sequence. As dsRNA is not a normal component of the transcriptome, it is believed that RNAi is part of a genome surveillance system that is used to repress transposons and viruses that have dsRNA as part of their replication cycle. Similar short sequences of RNA, called microRNAs, repress translation when they are bound to almost-complementary sequences in the 3 untranslated region of mRNA. 

RNAi is a naturally occurring cellular mechanism for regulating gene expression mediated by siRNAs. The ability to selectively degrade the mRNA encoding a disease-related protein has provided the opportunity to create a novel therapeutic modality. The process of candidate selection for... 

The mRNA for L chains has been reported to contain 850 (21), 1250 (26), or 1300 (28a) bases, rather than the 640 required to encode an L chain. Part of the difference is poly(A). In addition the polypeptide synthesized in vitro is actually larger than an L chain by at least 20 amino acids a degradative process would be required to produce the L chain (19,21,24a,25,27a,27c).2... 

The short length of a typical ASON facilitates cell internalization and increases hybridization efficiency by reducing base-mismatch errors. Once hybridization has occurred the ASON-mRNA complex becomes a substrate for intracellular RNAses (e.g., RNAse-H) that catalyze mRNA degradation and allow ASON to recycle for another base pairing with the next target mRNA molecule. The net result of this process is a sustained decrease of target mRNA translation and a lower intracellular level of the corresponding protein (Fig. 1). 

Figure 43-3. The "information pathway." Information flows from the gene to the primary transcript to mRNA to protein. Hormones can affect any of the steps involved and can affect the rates of processing, degradation, or modification of the various products.

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