Quality control of newly synthesized mRNA is performed by decapping exoribonuclease (DXO)

 Newly synthesized mRNA is readily capped and methylated by a capping enzyme and cap methyltransferase respectively, both of which directly interact with RNA polymerase II. Since the mRNA cap plays pivotal rolls in mRNA lifecycle including splicing, polyadenylation, mRNA export and translation, the incomplete capped mRNA should be removed immediately after its appearance. Recently, the partial mechanism of the degradation of incomplete capped mRNA has been reported, which is controlled by an enzyme, decapping exoribonuclease (DXO). DXO shows three enzymatic activities: I. pyrophosphohydrolase, II. decapping and III. 5'-to-3' exoribonuclease.

 I. pyrophosphohydrolase:   pppRNA -> pp + pRNA
 II. decapping:                      GpppNpRNA -> GppN + pRNA
 III. 5'-to-3' exoribonuclease: RNA degradation from 5' to 3' direction

 If a mRNA is recognized as an incomplete form such as pppRNA or nonmethylated GpppRNA, DXO removes the pp or entire cap structure (GppN) from the 5' terminal of mRNA to generate pRNA that serves as a substrate for the exoribonuclease activity to degrade the RNA body. It is considered that cells control the quality of newly synthesized mRNA by deleting incomplete mRNAs, and prevent from accumulation of abnormal RNAs.

http://www.sciencedirect.com/science/article/pii/S1097276513001731