Structural comparison of 37-S and 32-S ribosomal precursor RNA in yeast with the mature ribosomal RNA components

Secondary methylation of yeast ribosomal precursor RNA

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 psi 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 to the cytoplasm prior to the final maturation into 17-S rRNA.

The molecular weights of yeast ribosomal precursor RNAs

The molecular weights of the predominant rRNA precursors as well as those of 26-S and 17-S mature rRNA from Saccharomyces carlsbergensis were determined by polyacrylamide gel electrophoresis in the presence of formamide. Mature 26-S + 5.8-S rRNA was found to have a molecular weight of 1.24 X 10(6) while their immediate precursor, 29-S RNA, had a molecular weight of 1.52 X 10(6). Values of 0.70 X 10(6) and 0.82 X 10(6) were obtained for the molecular weights of mature 17-S rRNA and its 18-S precursor. Finally the 37-S precursor, common to both 29-S and 18-S RNA, was found to have a molecular weight of 2.80 X 10(6). Each precursor rRNA, therefore, contains extra sequences not found at the next stage of maturation.