Partial mapping of methylated sequences in Xenopus laevis ribosomal RNA by preparative hybridization to cloned fragments of ribosomal DNA.

Xenopus laevis rRNA was hybridised to either of two cloned fragments of ribosomal DNA. One fragment, designated X1r11, contains a short region of the 18 S rRNA gene and most of the 28 S rRNA gene. The other fragment, X1r14, contains a short region of the 28 S gene and most of the 18 S gene. After hybridization the non-complementary rRNA was removed by digestion with T1 RNase and the hybridized RNA was then eluted and examined by fingerprinting analysis. The 3' terminal sequence and the dimethyl-A-containing sequence of 18 S rRNA both hybridized to X1r11 rDNA, in agreement with the known direction of transcription of rDNA. The distribution of other methylated oligonucleotides between the various fingerprints permitted assigment of nearly all of the methylated sequences in 18 S and 28 s rRNA to either the short 3' region or the long 5' region of the respective molecules.