Posttranscriptional processes

Poly(A) tails are added to the S end of mRNA molecules in a posttranscriptional processing step. The mRNA is first cleaved about 20 nucleotides downstream from an AAUAA recognition sequence. Another enzyme, poly(A) polymerase, adds a poly(A) tail which is subsequently extended to as many as 200 A residues. The poly(A) tail appears to protect the S end of mRNA from S —> S exonuclease attack. The presence or absence of the poly(A) tail does not determine whether a precursor molecule in the nucleus appears in the cytoplasm, because all poly(A)-tailed hnRNA molecules do not contribute to cytoplasmic mRNA, nor do all cytoplasmic mRNA molecules contain poly(A) tails... 

Increased transcription levels are assumed to result in increased protein synthesis. One approach to reach this goal is to raise the transgene copy number by the use of amplification-promoting sequences derived from a spacer sequence of tobacco ribo-somal DNA [95]. Posttranscriptional processes such as capping, splicing and polya-denylation are important for high protein yields, and it is also important to maximize mRNA stability [84]. 

The primary transcript must undergo extensive posttranscriptional processing inside the nucleus to form the mature mRNA molecule (F ure 1-3-7). These processing steps indude the fol-... 

Posttranscriptional processing is not limited to mRNA. Ribosomal RNAs of both prokaryotic and eukaryotic cells are made from longer precursors called preribosomal RNAs, or pre-rRNAs, synthesized by Pol I. In bacteria, 16S, 23S, and 5S rRNAs (and some tRNAs, although most tRNAs are encoded elsewhere) arise from a single 30S RNA precursor of about 6,500 nucleotides. RNA at both ends of the 30S precursor and segments between the rRNAs are removed during processing (Fig. 26-21). 

Transfer RNA precursors may undergo further posttranscriptional processing. The 3 -terminal trinucleotide CCA(3 ) to which an amino acid will be attached during protein synthesis (Chapter 27) is absent from some bacterial and all eukaryotic tRNA precursors and is added during processing (Fig. 26-23). This addition is carried out by tRNA nucleotidyltransferase, an unusual enzyme that binds the three ribonucleoside triphosphate precursors in separate active sites and catalyzes formation of the phosphodiester bonds to produce the CCA(3 ) sequence. The creation of this defined sequence of nucleotides is therefore not dependent on a DNA or RNA template—the template is the binding site of the enzyme. 

The study of posttranscriptional processing of RNA molecules led to one of the most exciting discoveries in modern biochemistry—the existence of RNA enzymes. The best-characterized ribozymes are the self-splicing group I introns, RNase P, and the hammerhead ribozyme (discussed below). Most of the activities of these ribozymes are based on two fundamental reactions transesterification (Fig. 26-13) and phosphodiester bond hydrolysis (cleavage). The substrate for ribozymes is often an RNA molecule, and it may even be part of the ribozyme itself. When its substrate is RNA, an RNA cat-... 

RNA Posttranscriptional Processing Predict the likely effects of a mutation in the sequence (5 )AAUAAA in a eukaryotic mRNA transcript. 

The pattern of exons and introns (exon-intron-exon-intron-exon) in the a and /3 families of genes is quite old and apparently developed before the separation of these genes some 500 million years ago. The role of introns is unknown, although their base sequences vary. Because the introns are not transcribed, the protein sequences are unchanged. The introns appear to correspond to structural domains within the folded globulin subunits and are requisite for the posttranscriptional processing of mRNA. 

The chemistry of RNA synthesis is identical for all forms of RNA, including messenger RNA, transfer RNA, and ribosomal RNA. The basic steps just outlined also apply to all forms. Their synthetic processes differ mainly in regulation, posttranscriptional processing, and the specific polymerase that participates. 

Li W, Ishida T, Tachibana R, Almofti MR, Wang X, Kiwada H (2004) Cell type-specific gene expression, mediated by TFL-3, a cationic liposomal vector, is controlled by a posttranscription process of delivered plasmid DNA. Int J Pharm 276 67-74... 

Elferink, M.G. et al. (2004) I PS-induced downregulation of MRP2 and BSEP in human liver is due to a posttranscriptional process. American Journal of Physiology. Gastrointestinal and Liver Physiology, 287... 

The events in posttranscriptional processing of mRNA are controlled by the phosphorylation state of the carboxy-terminal domain (CTD), part of RNA polymerase II. 

Each rRNA operon encodes a primary transcript that contains one copy each of 16S, 23S, and 5S rRNAs. Each transcript also encodes one or two spacer tRNAs and as many as two trailer tRNAs. Posttranscriptional processing involves numerous cleavage reactions catalyzed by various RNases and splicing reactions. (Individual RNases are identified by letters and/or numbers, e.g., M5, X, and HI.) RNase P is a ribozyme. 

Posttranscriptional processing of tRNA requires several distinct steps, as summarized in Figure 25.8. First, the 50 and 30 ends must be cleaved to release the tRNA sequence from the larger precursor transcript and introns must be removed if they are present. Second, the required CCA charging sequence at the 30 end of tRNA must sometimes be added by a nucleotidyl transferase. Third, all tRNAs contain a large number of modified bases which result from reductions, methylations, and deaminations. These modifications can affect codon recognition by the tRNAs during protein synthesis (Chapter 26). 

Posttranscriptional processing of primary RNA transcripts includes RNA splicing, 50 capping, 30 polyadenylation, and tRNA base modifications. Many of these alterations increase RNA stability and enhance inRNA translation. 

See also Translation Overview, Eukaryotic vs Prokaryotic Translation, Figure 27.15, Stringent Response, Posttranscriptional Processing of rRNA and tRNA... 

An additional posttranscriptional process, namely intron splicing, is almost exclusively confined to... 

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