MESSENGER RNA PROCESSING

Part of “CHAPTER 3 – BIOSYNTHESIS AND SECRETION OF PEPTIDE HORMONES“

The hnRNA product of gene transcription is rapidly processed in the nucleus with a half-time (t_1/2) of 5 to 20 minutes (Fig. 3-8).22 Three major events transform the large heterogeneous RNA precursors into the mature RNA. First, at the 5′ end of hnRNA, a ⁷methylguanosine residue is added to the first nucleotide of the transcript by means of a 5′-5′ triphosphate bond after 20 to 30 nucleotides have been polymerized. This reaction is rapid (t_1/2 < 1 minute) and is catalyzed by the 5′-capping enzyme, including guanylyl and methyl transferases. The 5′-methyl cap associates with 5′-cap binding proteins, which favors the formation of a stable 40S translation-initiation complex and increases the stability and efficiency of translation of the eventual mature mRNA.23

FIGURE 3-8. Gene transcription and RNA processing. The initial RNA transcript is known as heterogeneous nuclear RNA (hnRNA I). It contains exons and introns of the structural region and rapidly undergoes 5′ capping with ⁷methylguanosine (⁷meG) and 3′ polyadenylation (A_n) (hnRNA II). Little heterogeneous nuclear RNA has been detected without 5′ cap or 3′ polyadenylated (poly[A]) tails. In a slower process, introns are removed by RNA splicing followed by religation of exon sequences. The mature messenger RNA (mRNA) is composed of fused exon sequences and contains a 5′ cap and a 3′ poly(A) tail.

Only gold members can continue reading. Log In or Register to continue