Scheme 1.

Schematic representation of 5′-cap methylation by the means of different methyltransferases. RNA equipped with a cap0 structure, prepared either chemically or enzymatically, can be further methylated by cap-modifying methyltransferases. The N²-methyltransferases GlaTgs2 and hTgs1convert the N⁷-methyl guanosine into the di-or tri-methyl mRNA caps ^2m,7mGppp or ^2m,2m,7mGppp, respectively. The first transcribed nucleotide can be methylated at its 2′-position by the human 2′-O-methyltransferase CMTR1.