An evolutionary study of the methylation of transfer and ribosomal ribonucleic acid in prokaryote and eukaryote organisms.

The methylated bases and nucleosides of transfer RNA and ribosomal RNA isolated from a number of eukaryote and prokaryote organisms were compared. Of the many different types of methylated bases and nucleosides found, only certain ones ever appeared in each type of methylated RNA; thus, the methylation of tRNA and both types of rRNA was a highly specific process. In addition, in all cells tested the pattern of methylation for tRNA and each rRNA differed markedly from each other. Eukaryote tRNA, whether isolated from yeast, amoeba, chicken, mouse, monkey, or human cells, always exhibited a strikingly similar pattern of methylation. In addition, a second and third pattern of methylation were found to be common to the lower and higher molecular weight species of ribosomal RNA, respectively, in the eukaryote cells tested. Each of these three eukaryote RNA species contained at least one characteristic methylated base not found (or found only in trace amounts) in any of the others. These were N2-dimethyIguanine and I-methyIhypoxanthine in tRNA, Nzdimethyladenine in 18 S RNA (16 S in yeast), and 3-methyluracil in 28 S RNA (25 S in yeast). The patterns of methylation of prokaryote tRNA and rRNA were different from those of their eukaryote RNA counterparts. Two gram-negative bacteria, Escherichia coli B and Pseudomonas aeruginosa, had identical methylation patterns for tRNA, 16 S RNA, and 23 S RNA, respectively. In addition, each of the ribosomal RNA species had a characteristic methylated base not found in the other. These were NCdimethyladenine in 16 S RNA, and N6-methyladenine in 23 S RNA. The pattern of methylation of tRNA from gramnegative bacteria, gram-positive bacteria, and mycoplasma differed only slightly from each other. It was concluded that great differences exist in the methylation patterns of the various RNA species in a given cell while great similarities exist in the methylation patterns of each RNA species throughout a wide range of eukaryote or prokaryote cells.