Poly tail hnRNA processing

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... 

Fig. 14.10. Overview of mRNA synthesis. Transcription produces hnRNA from the DNA template. hnRNA processing involves addition of a 5 -cap and a poly(A) tail and splicing to join exons and remove introns. The product, mRNA, migrates to the cytoplasm, where it will...

Small nuclear RNAs (snRNAs), a subset of these RNAs, are significantly involved in mRNA processing and gene regulation. Of the several snRNAs, Ul, U2, U4, U5, and U6 are involved in intron removal and the processing of hnRNA into mRNA (Chapter 37). The U7 snRNA may be involved in production of the correct 3 ends of histone mRNA—which lacks a poly(A) tail. The U4 and U6 snRNAs may also be required for poly(A) processing. 

The relationship between hnRNA and the corresponding mature mRNA in eukaryotic cells is now apparent. The hnRNA molecules are the primary transcripts plus their early processed products, which, after the addition of caps and poly(A) tails and removal of the portion corresponding to the introns, are transported to the cytoplasm as mature mRNA molecules. 

Posttranscrip-tional processing of hnRNA (pre-mRNA) None In nucleus 5 cap (7-MeG) 3 tail (poly-A sequence) Removal of introns from hnRNA Alternative splicing yields variants of protein product... 

A. Both hnRNA and mRNA contain poly(A) tails, but hnRNA contains introns that are removed as it is processed to form mRNA. 

Fig. 14.4. A schematic view of a eukaryotic gene, and steps required to produce a protein product. The gene consists of promoter and transcribed regions. The transcribed region contains introns, which do not contain coding sequence for proteins, and exons, which do carry the coding sequences for proteins. The first RNA form produced is heterogenous nuclear RNA (hn RNA), which contains both intronic and exonic sequences. The hnRNA is modified such that a cap is added at the 5 end (cap site), and a poly-A tail added to the 3 end. The introns are removed (a process called splicing) to produce the mature mRNA, which leaves the nucleus to direct protein synthesis in the cytoplasm. Py is pyrimidine (C or T).

After the gene is transcribed (i.e., posttranscription), regulation can occur during processing of the RNA transcript (hnRNA) into the mature mRNA. The use of alternative splice sites or sites for addition of the poly(A) tail (polyadenylation sites) can result in the production of different mRNAs from a single hnRNA and, consequently, in the production of different proteins from a single gene. 

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