Secondary Methylation of Yeast Ribosomal Precursor RNA

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Abstract
The timing of methylation of the ribosomal sequences of ribosomal precursor RNA (pre‐rRNA) from the yeast Saccharomyces carlsbergensis was investigated by fingerprint analysis of the methylated oligonucleotides derived from the various precursors. From the total of 37 ribose and 6 base‐methyl groups found in 26‐S rRNA, the two copies of the base‐methylated nucleoside m3U as well as the doubly methylated sequence Um‐Gm‐Φ are not yet present in 37‐S RNA, the predominant common precursor of 26‐S and 17‐S rRNA. Introduction of these methyl groups into the ribosomal sequences appears to take place at the level of 29‐S pre‐rRNA, the immediate precursor to 26‐S rRNA. From the total of 18 ribose‐methylated and 6 base‐methylated nucleosides found in 17‐S rRNA, the latter group (one copy of m7G, the ‐m62A‐m62A‐ sequence and the hypermodified methylated nucleoside ‘mX’) is completely missing in 37‐S pre‐rRNA. The methyl group of m7G is introduced into 18‐S pre‐rRNA, the direct precursor of 17‐S rRNA, in the nucleus. The ‐m62A‐m62A‐ sequence is methylated after transport of the 18‐S pre‐rRNA molecules to the cytoplasm prior to the final maturation into 17‐S rRNA.

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