Capping structures at the 5'-terminus of polyadenylated ribonucleic Acid in Avena coleoptiles.

Evidence is presented for the occurrence of 5'-terminal capping structures in the polyadenylated RNA of oat (Avena sativa) coleoptiles. These structures are composed of an inverted terminal nucleoside containing the modified base 7-methylguanine which is joined 5' to 5' with a second (penultimate) nucleoside by means of three phosphate groups in two pyrophosphate linkages. The penultimate nucleoside is joined to the remainder of the RNA molecule by a conventional 3',5' phosphodiester bond. A significant difference between the cap structures of oat coleoptile RNA and those of previously described higher eucaryotic cellular mRNAs is the lack of ribose methylations in the penultimate nucleosides of the plant RNA.