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Messenger RNA and hnRNA both have polv(A) tails
Update time:2018-12-25 21:56:29   【 Font: Large  Medium Small

    The next major advance in studying eukaryotic mRNA synthesis took place in the early 1970s as a result of efforts to characterize HeLa cell mRNA by digesting it with ribonucleases. The mRNA was extracted from cytoplasmic complexes known as polyribosomes or polysomes, each consisting of two or more ribosomes translating a single mRNA. Two different endonucleases, RNase TI (a product of the fungus Aspergillus oryzae) and pancreatic RNase, were used to digest the mRNA. RNase TI and pancreatic RNase cleave phosphodiester bonds on the 3’-sides of guanylate residues and pyrimidine nucleotide distribution within HeLa cell mRNA, one would expect to find the digests would contain a mixture of short oligonucleotides along with some mononucleotides. Although digesting HeLa cell mRNA with either T1 RNase or pancreatic RNase did indeed produce the expected digestion products, it also produced an entirely unexpected polynucleotide containing 150 to 200 adenylate groups. Further analysis showed that these poly (A) segments are attached to the 3’-end of mRNA. With the one notable exception of histone mRNA, all eukaryotic mRNAs have such 3'-poly (A) tails. Yeast poly (A) tails are usually between 50 and 70 nucleotides long, considerabiy shorter than mammalian poly (A) tails. Subsequent studies showed that a large proportion of the rapidly labeled hnRNA molecules also have poly (A) tails, supporting the idea that these hnRNA molecules are converted to mRNA.
    Poly (A) tails have a practical laboratory application; their baes paring properties can be uesd to separate mRNA and pre-mRNA from other kinds of RNA. This separation is accomplished by passing a mixture of RNA molecules, dissolved in a buffer solution containing a high salt concentration, through a column packed with cellulose fibers linked to oligo (dT). Messenger RNA molecules stick to the column because of base pairing between their poly (A) tails and oligo (dT). The same is true for pre-mRNA molecules. Metal cations from the salt help to stabilize this base pairing by diminishing charge repulsion between phosphate groups. Other kinds of RNA molecules that lack poly (A) tails pass right through the column. Then mRNA molecules (or pre-mRNA molecules) are eluted from the column by washing with a low salt buffer. It is important to note that not all of the RNA molecules in the rapidly labeled hnRNA fraction bind to oligo (dT). Nearly all of those that do not bind are destined to become some other kind of cellular RNA such as ribosomal or transfer RNA.

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