For example, the interferon-induced ISG20 3вЂ™exo has anti-viral activity. Finally, other 3вЂ™-to-5вЂ™ exonucleases also contribute to specialized mRNA decay mechanisms. The exosome is also involved in the degradation of the extensive array of cryptic nuclear RNA polymerase II transcripts that have recently been described. In addition to its contribution to general mRNA stability, the exosome also contributes to regulated mRNA decay mediated by AU-rich elements as well as to quality control of gene expression by degrading defective transcripts, such as mRNAs that have premature translation termination codons, or those which lack termination codons altogether. The mRNA body undergoes degradation either by the exosome (3вЂІ-to-5вЂІ pathway) or by cleavage of the 5вЂІ cap followed by 5вЂІ-to-3вЂІ degradation by the Xrn1p exoribonuclease (5вЂІ-to-3вЂІ pathway). In eukaryotic cells, the degradation of most mRNAs starts with deadenylation, which is performed by a variety of poly(A)-specific 3вЂ™ exos. 3вЂ™hExo was initially thought to be involved in degradation of these transcripts, but it now seems more likely that it is required for cytoplasmic 3вЂ™ trimming. ДЇinally, a 3вЂ™ exo, 3вЂ™hExo, has recently been found associated with histone mRNAs. coli, 3вЂ™ exos, particularly RNase T and RNase PH, are essential for 3вЂ™ maturation of tRNA and other small RNAs. In Pseudomonas syringae, RNase R is involved in 3вЂ™ trimming of the 16S and 5S rRNAs whilst in E. Likewise, in prokaryotes, 3вЂ™exos have been shown to play a role in processing of small stable RNAs, and rRNA. In the pathway of rRNA maturation, for example, exosome mutations directly lead to elevated levels of the 5вЂ™ external transcribed spacer region and indirectly to the reduction of cleavage efficiency at a variety of steps in the processing of the large ribosomal RNAs. Finally, the exosome contributes directly or indirectly to a variety of RNA processing events. Interestingly, the exosome also contributes to an alternative pathway of 3вЂ™ end formation for mRNAs. U14, U18, U24 that are excised from larger precursors) Other 3вЂ™ exos may also contribute to the 3вЂ™ end trimming of 5S, U4 and MRP RNAs. Deletion or mutation of exosome components leads to accumulation of transcripts with extended 3вЂ™ ends including 5.8S rRNA, snRNAs (e.g. Since transcription termination does not occur precisely at the 3вЂ™ end of mature transcripts, this activity contributes to the 3вЂ™ end trimming/processing of several classes of RNAs in the nucleus. In eukaryotic cells, the vast majority of 3вЂ™ exo activity is contributed by the exosome, a complex of nine core and several auxiliary components. Given that the exosome, a large complex of exoribonucleases present in archaea and eukaryotes, is perhaps the most prolific 3вЂ™-to-5вЂ™ exonuclease activity known, we will devote a significant proportion of our discussion to this enzyme complex. In this review, we will discuss the various characterized 3вЂ™-to-5вЂ™ exonuclease activities, their substrate spec ificities and regulation. Most will recognize only single-stranded 3вЂ™ RNA ends, others have sequence specificity, and often stimulatory co-factors are closely associated with the core enzymatic activity.

Not surprisingly, given the destructive activity of these enzymes, they experience various regulatory restrictions. 3вЂ™ exos have been implicated in many such processing events. Finally, transcription does not terminate precisely at the 3вЂ™ end of a transcript, thus 3вЂ™ processing is essential to the maturation of both mRNAs and non-coding RNAs. Several 3вЂ™ exos are essential to the removal of these aberrant transcripts from the cell. Second, aberrant RNA molecules are inevitably generated during transcription and processing. Importantly, 3вЂ™-to-5вЂ™ exonucleases (3вЂ™ exos) play both general and regulated roles in RNA decay.

Thus, RNA turnover rates are an integral component of the control of gene expression.

RNA levels are determined not only by their rate of synthesis, but also by the rate of degradation. First, exonucleases are vital for many pathways of RNA decay. Exoribonuclease activities play major roles in both prokaryotic and eukaryotic cells.

Core process 3 5 activities