Figure 1. Cap adjacent modification in higher Eukaryotes.

Formation of the cap1 structure takes place by 2'-O –methylation on the ribose residue on the first nucleotide adjacent to the m⁷G cap. The example shows a structure with adenosine as the first nucleotide, adjacent to the m7G cap (a). The cap1 structure can then be converted to a cap2 by a further 2'-O-ribose methylation on the following nucleotide (b), or converted to a cap1 m6Am by placing a further methyl group on the adenosine in the N⁶ position (c). The figure is a schematic representation of the mRNA cap structures. The yellow structure represents 7-methyl-guanine, the green represents adenine and the grey pentagon is ribose. On the ribose only the methylated functional groups are shown.