| Abstract: | Crude tRNA isolated from rat liver by the method of Rogg et al. (Biochem. Biophys. Acta 195, 13-15 1969) contains N6-dimethyladenosine (m6-2A) and was therefore fractionated in order to identify the m6-2A-containing RNAs. A unique species of RNA was purified which contained all the m62A present in the crude tRNA. Sequence analysis by postlabeling with gamma-32p-ATP and polynucleotide kinase revealed that this RNA represents the 32 nucleotides AAGGUUUC(C)U GUAGGUGm62Am62ACCUGCGGAAGGAUC from position 5 to 36 of the 3' terminus of ribosomal 18S RNA. The 36 nucleotide long sequence from the 3' end of rat liver 18S rRNA exhibits extensive homology with the corresponding sequence of E. coli 16S rRNA and with the 21 nucleotide long 3' terminal sequence so far known from Saccharomyces carlsbergensis 17S rRNA. A heterogeneity in this sequence provides the first evidence on the molecular level for the existence of (at least) two sets of redundant ribosomal 18S RNA genes in the rat. |
