DNA-methylation in mouse embryo development : R. Balling, R. Kothary, J. Rossant, D. Huszar and A. Bernstein, Dept. of Biol. Sci., Brock Univ., St. Catharines, Ont., L253A1, and Dept. of Med. Biophys., U. of Toronto, Ont. M4X1

Abstract

1. The minor nucleotide composition of the “ribosomal precursor” RNA of Escherichia coli has been compared with that of mature ribosomal RNA. When cells were treated with chloramphenicol, or with streptomycin, or when a “relaxed” auxotroph was starved of a required amino acid, the newly synthesized, “ribosomal precursor”, 16-S RNA (centrifugally purified) was found to be undermethylated by 70–80 %, and 23-S RNA by 40–45 %. Nucleotide analyses showed that the 23S precursor component was about equally deficient in all normal methylated nucleotides, except for an O′-ribose-methylated nucleotide, whose accumulation was more drastically affected. The 16S precursor component had virtually no methylated nucleotides, except for approximately one 5-methylcytidylic acid residue per molecule. These results are in accord with the idea that undermethylation causes previously observed centrifugal and chromatographic abnormalities of ribosomal precursor RNA.2. Chloramphenicol treatment was also found to inhibit the formation of pseudouridylic acid in newly synthesized ribosomal RNA.3. No differences in ribosomal RNA methylation patterns were noted among strains B, W, or K12, or between streptomycin-sensitive and streptomycin-resistant strains.4. A model is presented whereby certain “structural” ribosomal proteins also function as ribosomal RNA-modifying enzymes during ribosome assembly.