Wheat embryo ribonucleates. XIII. Methyl-substituted nucleoside constituents and 5′-terminal dinucleotide sequences in bulk poly(A)-rich RNA from imbibing wheat embryos

Abstract

(1) N 6 -Methyladenosine, not previously detected as a component of plant mRNA, is shown to be present in the poly(A)-rich RNA which is isotopically labeled when wheat embryos imbibe water in a medium which contains [methyl- 3 H]methionine.(2) N 6 -Methyladenosine and 7-methylguanosine 'caps' can constitute more than 85% of the radioactivity when specimens of poly(A)-rich [methyl- 3 H]RNA are freed of most of the radioactivity contributed by contaminating rRNA, N 6 -methyladenosine accounting for 75–80% and 7-methylguanosine accounting for 20–25% of this radioactivity.(3) There are three principal 5′-terminal dinucleotide sequences (type 0 'cap' structures) in the poly(A)-rich RNA from imbibing wheat embryos: m 7 GpppA, m 7 GpppG, and m 7 GpppC.(4) The proportional distribution of radioactivity among the 5′-terminal dinucleotide sequences varies with the time of labeling of the imbibing wheat embryos (30 min and 24 h), m 7 GpppA being the most 'rapidly labeled' sequence and m 7 GpppC being the most 'slowly labeled' sequence.(5) Poly(A)-rich mRNA from imbibing wheat embryos is distinct from the corresponding RNA from any other organism yet studied in terms of its component methyl-substituted nucleosides and 5′-terminal dinucleotide sequences. The possible significance of these differences, as well as signal similarities between the poly(A)-rich mRNA of imbibing wheat embryos and other organisms, are subjects of discussion.