Apolipoprotein mRNA processing

The 5 ends of mRNA precursors become capped and methylated in the course of transcription. A 3 poly(A) tail is added to most mRNA precursors after the nascent chain has been cleaved by an endonuclease. RNA editing processes alter the nucleotide sequence of some mRNAs, such as the one for apolipoprotein B. 

Some cells have been observed to change the coding properties of newly synthesized mRNA molecules. In this process, called RNA editing, certain bases are chemically modified, deleted, or added. For example, the mRNA for apolipoprotein B-100 in liver cells codes for a 4563 amino acid polypeptide which is a component of very low density lipoprotein (VLDL). Intestinal cells produce a shorter version of the molecule called apolipoprotein B-48 (2153 amino acid residues) that becomes incorporated into the chylomicron particles produced by these cells. The cytosine in a CAA codon that specifies glutamine is converted by a deamination reaction into a uracil. The new codon, UAA, is a stop signal in translation hence a truncated polypeptide is produced during translation of the edited mRNA. 

---