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

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Abstract
(1) N6-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-3H]methionine.(2) N6-Methyladenosine and 7-methylguanosine 'caps' can constitute more than 85% of the radioactivity when specimens of poly(A)-rich [methyl-3H]RNA are freed of most of the radioactivity contributed by contaminating rRNA, N6-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: m7GpppA, m7GpppG, and m7GpppC.(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), m7GpppA being the most 'rapidly labeled' sequence and m7GpppC 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.

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