<Li> Splicing removes the introns, noncoding regions that are transcribed into RNA, in order to make the mRNA able to create proteins . Cells do this by spliceosomes binding on either side of an intron, looping the intron into a circle and then cleaving it off . The two ends of the exons are then joined together . </Li> <Li> Addition of poly (A) tail otherwise known as polyadenylation . That is, a stretch of RNA that is made solely of adenine bases is added to the 3' end, and acts as a buffer to the 3' exonuclease in order to increase the half life of mRNA . In addition, a long poly (A) tail can increase translation . Poly (A) - binding protein (PABP) binds to a long poly (A) tail and mediates the interaction between EIF4E and EIF4G which encourages the initiation of translation . </Li> <Li> RNA editing is a process which results in sequence variation in the RNA molecule, and is catalyzed by enzymes . These enzymes include the Adenosine Deaminase Acting on RNA (ADAR) enzymes, which convert specific adenosine residues to inosine in an mRNA molecule by hydrolytic deamination . Three ADAR enzymes have been cloned, ADAR1, ADAR2 and ADAR3, although only the first two subtypes have been shown to have RNA editing activity . Many mRNAs are vulnerable to the effects of RNA editing, including the glutamate receptor subunits GluR2, GluR3, GluR4, GluR5 and GluR6 (which are components of the AMPA and kainate receptors), the serotonin2C receptor, the GABA - alpha3 receptor subunit, the tryptophan hydroxlase enzyme TPH2, the hepatitis delta virus and more than 16% of microRNAs . In addition to ADAR enzymes, CDAR enzymes exist and these convert cytosines in specific RNA molecules, to uracil . These enzymes are termed' APOBEC' and have genetic loci at 22q13, a region close to the chromosomal deletion which occurs in velocardiofacial syndrome (22q11) and which is linked to psychosis . RNA editing is extensively studied in relation to infectious diseases, because the editing process alters viral function . </Li> <Li> mRNA Stability can be manipulated in order to control its half - life, and the poly (A) tail has some effect on this stability, as previously stated . Stable mRNA can have a half life of up to a day or more which allows for the production of more protein product; unstable mRNA is used in regulation that must occur quickly . </Li>

Which of the following are forms of post transcriptional regulation in eukaryotes