RNA processing

All classes of RNA transcripts must be processed into mature species. The reactions include several types Nucleolytic cleavage, as in the separation of the mature rRNA species from the primary transcript of RNA polymerase I action Chain extension (non-template-directed), as in the synthesis or regeneration of the common CCA sequence at the 3 end of transfer RNAs or of PolyA at the 3 end of mRNAs and Nucleotide modification, for example, the synthesis of methylated nucleotides in tRNA or rRNA. These reactions are a feature of both prokaryotic and eukaryotic gene expression, and the biological consequences are diverse. For example, modified nucleotides can affect the way in which a tRNA recognizes different codons. 

Notes Ranked in order of the ratio of the most frequent count in a loop over the least. 

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In all eukaryotic cells there are small nuclear RNA (snRNA) species that are not directly involved in protein synthesis but play pivotal roles in RNA processing. These relatively small molecules vary in size from 90 to about 300 nucleotides (Table 35-1). 

After transcription, during RNA processing, introns are removed and the exons are ligated together to form the mamre mRNA that appears in the cytoplasm. 

Figure 37-13. Mechanisms of alternative processing of mRNA precursors. This form of RNA processing involves the selective inclusion or exclusion of exons, the use of alternative 5 donor or 3 acceptor sites, and the use of different polyadenylation sites.

In addition to affecting the efficiency of promoter utilization, eukaryotic cells employ alternative RNA processing to control gene expression. This can result when alternative promoters, intron-exon splice sites, or polyadenylation sites are used. Occasionally, heterogeneity within a cell results, but more commonly the same primary transcript is processed differendy in different tissues. A few examples of each of these types of regulation are presented below. 

Gene expression can be controlled at multiple levels by changes in transcription, RNA processing, localization, and stabihty or utilization. Gene amphfica-tion and rearrangements also influence gene expression. 

Point mutations Protein folding Transcriptional control Frameshiftand nonsense mutations RNA processing Sickle cell disease P-Thalassemia P-Thalassemia P-Thalassemia... 

Interestingly, Chisaka et al. (1992) targeted the Hox-1.6 gene in a different manner and observed some distinct phenotypic effects. Alternate RNA processing allows the Hox-1.6 gene to encode two different proteins, one with the homeodomain and one without it. The experiments... 

Blumenthal, T. and Steward, K. (1997) RNA processing and gene structure. In Riddle, D.L., Blumenthal, T., Meyer, B.J. and Priess,J.R. (eds) C. ehgansII Cold Spring Harbor Laboratory Press, Cold Spring Harbor, New York, pp. 117-145. 

Degradation of RNA DNA-dependent RNA polymerase Transcription factors RNA processing Other Translation... 

Ginisty H, Amalric F, Bouvet P (1998) Nucleolin functions in the first step of ribosomal RNA processing. Embo J 17 1476-1486... 

Hayward-Lester A, Chilton BS, Underhil PA, Oefher PJ, Doris PA. 1997. Quantitation of spedfic nudeic adds, regulated RNA processing and genomic polymorphism using reversed-phase HPLC. Gene quantification. Ferre F, editor. Boston Birkhauser. 

Xd>ie 1-3-2. Summary of Important Points About Transcription and RNA Processing... 

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