MRNA transcripts processing/modification

These processes are summarized in Figure 28-1. We have examined several of these mechanisms in previous chapters. Posttranscriptional modification of mRNA, by processes such as alternative splicing patterns (see Fig. 26-19b) or RNA editing (see Box 27-1), can affect which proteins are produced from an mRNA transcript and in what amounts. A variety of nucleotide sequences in an mRNA can affect the rate of its degradation (p. 1020). Many factors affect the rate at which an mRNA is translated into a protein, as well as the posttranslational modification, targeting, and eventual degradation of that protein (Chapter 27). 

In prokaryotes, RNA transcribed from protein-coding genes (messenger RNA, mRNA), requires little or no modification prior to translation. In fact, many mRNA molecules begin to be translated even before RNA synthesis has finished. However, ribosomal RNA (rRNA) and transfer RNA (tRNA) are synthesized as precursor molecules that do require post-transcriptional processing (see Topics G9 and G10, respectively). 

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. 

Prokaryotic ribosomes attach to the nascent mRNA while it is still being transcribed. Because transcription and translation are coupled, prokaryotic mRNAs undergo little modification and processing before being used as templates for protein synthesis. Prokaryotic tRNA and rRNA are transcribed in units larger than those ultimately used and must be processed to generate the functional molecules. The processing of these and the eukaryotic primary transcripts, almost all of which require modification, is discussed in a later section. 

No. In the vast majority of cases, the mRNA, which is always monocistronic in eukaryotes, is formed by the processing (modification and splicing) of primary transcripts. 

In higher organisms, a gene is first transcribed into a transcript or pre-mRNA. The latter undergoes additional modifications called processing to produce translatable mRNA. The processing involves at least three steps. The first step includes a cap or the addition of novel guanosine nucleotide at the 5 end, and the second step includes a tail or the addition of a poly A nucleotides at the 3 end. The third step is the removal of... 

Messenger RNA processing/modification. Most primary mRNA transcripts in prokaryotes function in translation without further modification. Eukaryotic mRNA transcripts, however, undergo extensive posttranscriptional modifications. 

Transcription produces RNA molecules. In prokaryotes, rRNAs and tRNAs are synthesized as longer precursors which require post-transcriptional processing including methylation and cleavage by specific endonucleases before they are functional molecules. Polycistronic mRNAs (Section 7.4) do not require further enzymic modification. 

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.

All four scientists whose work led to modification of the dogmas received the Nobel Prize. Thomas Cech (1987) was the first to observe enzyme-like reactions taking place at the same RNA strand, in ribosomal RNA (rRNA) from the proto-zoon Tetrahymena thermophila. The RNA produced, which is completely viable, is formed in a process in which certain sections (introns) of the primary copy (the transcription of DNA to mRNA) are cut out, the two remaining ends of the exon then being rejoined (spliced). 

Transcription is the term used to describe the synthesis of RNA from a DNA template. Translation is the process by which information in RNA is used to synthesise a polypeptide chain. In a little more detail, the genetic information encoded in DNAis first transcribed into acomplementary copy of RNA (a primary RNA transcript) which is then processed to form messenger RNA (mRNA). This leaves the nucleus and is translated into a polypeptide in the cytosol. This then folds into a three-dimensional structure and may be further biochemically modified (post-transla-tional modification) to produce a protein (Figure 20.18). 

The molecular characterization of different 5-HT receptor subtypes has simplified fhe elucidation of gene transcription, mRNA processing, and translation as well as intracellular trafficking and posttranslational modification relevant to synaptic and postreceptor signaling. Transcriptional control regions... 

FIGURE 6.1 The different stages of protein synthesis. Transcription and processing of RNA messages occur within the nucleus. The mRNA is then transported into the cytoplasm for translation and post-translational modifications. 

The transport from nucleus to cytoplasm is accompanied by modification at the 5 - and 3 -end of the pre-RNA, as well as by processing (splicing) of the primary transcript. The 3 -end modifications and sphcing decide which information contained in the primary transcript is made available for protein biosynthesis. The information content of the processed mRNA can be specifically influenced by these processes. This has an important impact on the tissue- and cell-specific protein expression. 3 -modification and splicing are tightly coupled to extranuclear transport. Interventions in the transport process are another possibihty for a regulation at the post-transcriptional level. 

Stuart, K., RNA editing in mitochondrial mRNA of trypanoso-matids. Trends Biochem. Sci. 16 68-72, 1991. RNA editing is processing that involves the removal, addition, and modification of nucleotides in the coding regions of nascent transcripts. 

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