title: Kinetics of formation of 5' terminal caps in mRNA.
abstract: A kinetic analysis of the labeling of the methylated components of messenger RNA and heterogeneous nuclear RNA in mouse L cells indicates that the 5' terminal cap I structures (m7GpppXmpYp) of mRNA are derived from 5' terminal cap structures of hnRNA. Most of the hnRNA caps are conserved during processing, whereas only a portion of the internal m6A residues in hnRNA are conserved. The  cap II structures (m7GpppXmpYmpZp), which constitute the 5' termini of some mRNAs, arise by a "secondary" methylation that occurs after the mRNAs have entered the cytoplasm. This secondary methylation is apparently restricted to a particular subclass of mRNAs having a high frequency of pyrimidine nucleotides at position Y, a composition at position X which differs from that of the bulk of the cap I-terminated mRNAs, and a relatively slow rate of turnover.
mesh_terms: D007700:Kinetics; D007739:L Cells; D008715:Methionine / Q000378:metabolism*; D008745:Methylation; D011061:Poly A; D011132:Polyribosomes / Q000378:metabolism; D012313:RNA / Q000378:metabolism; D012333:RNA, Messenger / Q000032:analysis / Q000378:metabolism*
chemical_list: D012333:RNA, Messenger; D011061:Poly A; D012313:RNA; D008715:Methionine