Polyadenylation of RNA

Ara-A is phosphorylated in mammalian cells to ara-AMP by adenosine kinase and deoxycytidine kinase. Further phosphorylation to the di- and triphosphates, ara-ADP and ara-ATP, also occurs. In HSV-1 infected cells, ara-A also is converted to ara-ATP. Levels of ara-ATP correlate directly with HSV rephcation. It has recently been suggested that ara-A also may exhibit an antiviral effect against adenovims by inhibiting polyadenylation of viral messenger RNA (mRNA), which may then inhibit the proper transport of the viral mRNA from the cell nucleus. 

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.

The polyadenylation of c-mos mRNA in maturing oocytes is similar to that of other maternal RNAs that are accumulated during oocyte... 

Splicing is just one of the fonr processes that can modify the mRNA molecnle prior to transfer into the cytosol. The other three are cap formation, polyadenylation and editing of RNA. These are colectively known as RNA processing... 

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.

The first step in the replication of influenza viruses, which takes place in the cytoplasm, is the synthesis of (+) strands that can serve both as mRNA for synthesis of proteins and as templates for synthesis of new (-) strands. Three of the capsid proteins form the required RNA polymerase. This "transcriptase" is primed preferentially by 5 -capped 10- to 13-nucleotide segments of RNA that have been cut by a viral nuclease from host mRNAs.700 The mRNAs made from viral RNA are polyadenylated and are translated by the host cell s ribosomes. However, some transcripts are used as templates to form viral (-) strands, which... 

Jacobsen, J. V. Zwar, J. A. Gibberellic acid and RNA synthesis in barley aleurone layers metabolism of rRNA and tRNA and of RNA containing polyadenylic acid sequences. 

To demonstrate polymerase activity in a model cell, Chakrabarti et al. [79] encapsulated polynucleotide phosphorylase in vesicles composed of dimyris-toylphosphatidylcholine (DMPC). This enzyme can produce RNA from nucleoside diphosphates such as adenosine diphosphate (ADP) and does not require a template, so it has proven useful for initial studies of encapsulated polymerase activity (Fig. 10a). Furthermore, DMPC liposomes are sufficiently permeable so that 5-10 ADP molecules per second enter each vesicle. Under these conditions, measurable amounts of RNA in the form of polyadenylic acid were synthesized and accumulated in the vesicles after several days incubation. The enzyme-catalyzed reaction could be carried out in the presence of a protease external to the membrane, demonstrating that the vesicle membrane protected the encapsulated enzyme from hydrolytic degradation. Similar behavior has been observed with monocarboxylic acid vesicles [80], and it follows that complex phospholipids are not required for an encapsulated polymerase system to function. 

Matis, S., Xu, Y., Shah, M Guan, X., Einstein, J. R., Mural, R. Uberbacher, E. (1996). Detection of RNA polymerase II promoters and polyadenylation sites in human DNA sequence. Comput Chem 20,135-40. 

Figure 28.25. Polyadenylation of a Primary Transcript. A specific endonuclease cleaves the RNA downstream of AAUAAA. Poly(A) polymerase then adds about 250 adenylate residues.

Other a-globin mutants have a mutation at the intron-exon boundary (mRNA splice boundary) that prevents RNA processing or a mutation in the recognition sequence needed for polyadenylation of the mRNA during RNA maturation. In other a+-thalassemias with both a genes present but active only at a reduced level, the defect has not been identified. 

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