N6-adenosine methylation in mRNA: substrate specificity and enzyme complexity.

The N6-methylation of internal adenosine residues is a common post-transcriptional modification of eukaryotic pre-mRNA sequences. An in vitro methylation system which retains the precise selectivity of in vivo methylation sites has been used to further define the nature of RNA site recognition. In addition to short consensus sequences, other structural features or context effects contribute to the selection of methylation sites in pre-mRNAs. Partial purification of the mRNA N6-adenosine methyltransferase revealed unexpected levels of complexity. The methyltransferase is composed of three separate components with molecular masses of 30, 200 and 875 kDa, respectively. These components are readily separated under non-denaturing conditions and each is required for mRNA methylation activity.