Transcription polyadenylation

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. 

Figure 1. Expression of c-mos in mouse oocytes, c-mos is transcribed during oocyte growth and transcripts with short poly(A) tails are accumulated in fully-grown germinal vesicle (GV) stage oocytes. These transcripts are polyadenylated and translated following the resumption of meiosis and then degraded following fertilization and cleavage to the two-cell stage.

PolyA Signal This is the polyadenylation signal for attachment of the polyA tail to generate mature mRNA it is important for transcription termination. 

Figure 20.20 Summary of transcription, RNA processing and polypeptide synthesis. Polymerisation of the DNA template by RNA polymerase produces pre-mRNA (the primary transcript) this is transcription. The pre-mRNA is now processed, which involves capping, polyadenylation, editing and splicing (see text). The resultant mRNA transfers from the nucleus to the cytosol, where amino acids are polymerised to produce a polypeptide using the instructions present in the codons of the mRNA.

Transcription of genes in mammals often initially produces a pre-mRNA, whose information content can be modulated by subsequent polyadenylation or splicing. Various final mRNAs coding for proteins with varying fimction and localization can be produced in this manner starting from a single primary transcript. 

In higher eucaryotes the 3 -end of matme mRNA is not produced as a result of termination of transcription. Rather, the 3 -end of the primary transcript is cut at a specific site and a poly-A sequence is appended. Polyadenylation precedes the splicing of the primary transcript. 

During polyadenylation the primary transcript is shortened in an endonucleolytic step and appended with ca. 200 A-residues. The endonucleolytic incision requires two signal sequences on the pre-mRNA. A highly conserved AAUAAA sequence 10-30 nucleotides upstream from the hydrolysis site serves as one signal. Another signal in the form of a less well conserved GU- or U-rich element upstream of the hydrolysis site. Both together constitute the polyadenylation signal (Fig. 1.45). Polyadenylation occurs in a multiprotein complex, whose composition is not yet explained in all details. 

Fig. 1.46. Alternative polyadenylation in the expression of calcitonin genes of rat. The primary transcript of the calcitonin gene possesses two polyadenylation sites. One site is nsed in the processing of RNA in the thyroid, another site in the brain, and yet another in nerve tissne. The translation of the two mRNAs creates two pre-hormones, from which two different polypeptide hormones (calcitonin and the calcitonin-related peptide", or CGRP) are created via proteolysis.

Cai X, Hagedorn CH, Cullen BR. Human microRNAs are processed from capped, polyadenylated transcripts that can also function as mRNAs. RNA 2004 10 1957-1966. 

RNA is removed during processing. Pol II extends the primary transcript well beyond the cleavage and polyadenylation site ("extra RNA") before terminating transcription. Termination signals for Pol II have not yet been defined. 

FIGURE 26-19 Two mechanisms for the alternative processing of complex transcripts in eukaryotes, (a) Alternative cleavage and polyadenylation patterns. Two poly(A) sites, A, and A2, are shown. 

Addition of a poly-A tail Most eukaryotic mRNAs (with several notable exceptions, including those coding for the histones and some interferons) have a chain of 40 to 200 adenine nucleotides attached to the 3 -end (see Rgure 30.17). This poly-A tail is not transcribed from the DNA, but rather is added after transcription by the nuclear enzyme, polyadenylate polymerase. A consensus sequence, called the polyadenylation signal sequence (AAUAAA), found near the 3 -end of the RNA molecule, signals that a poly-A tail is to be added to the mRNA. These tails help stabilize the mRNAs and facilitate their exit from the nucleus. After the mRNA enters the cytosol, the poly-A tail is gradually shortened. 

A poly(A) "tail" consisting of -250 residues of adenylic acid is added next by poly(A) polymerase, a component of an enzyme complex that also cleaves the RNA chains.545 57111 Most eukaryotic mRNA is polyadenylated with the exception of that encoding histones. The function of the poly(A) is unclear. It is needed for transport of mRNA out of the nucleus, but it does confer a greatly increased stability to the mRNA in the cytoplasm where the adenylate irnits are gradually removed.307 308 In contrast, in chloroplasts and plant mitochondria polyadenylation is required for rapid degradation of mRNA.571c d Polyadenylation may also increase the efficiency of translation.572 Polyadenylation occurs rapidly within -1 min after transcription is completed. 

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