Kinetics of formation of 5' terminal caps in mRNA.

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.