Methylated nucleosides in globin mRNA from mouse nucleated erythroid cells.

Poly(A)-containing mRNAs labeled with [methyl-3H]methionine were isolated from nucleated erythroid cells obtained from the spleens of anemic mice. The RNAs were further separated into non-globin poly(A)-containing RNAs and highly purified globin mRNA by globin cDNA-cellulose affinity chromatography. DEAE-Sephadex column chromatography of the T2 ribonuclease digestion products of the cDNA-purified globin mRNA fraction yielded methylated resistant fragments with charges of -4.7 (Cap 1) and -5.3 (Cap 2). Digestion of the non-globin RNA fraction revealed a similar pattern with the addition of a methylated mononucleotide identified as 6-methyladenosine at -2 charges. Alkaline phosphatase treatment of the T2 resistant fragments reduced their charges by approximately 2, which is consistent with the removal of one terminal phosphate. Treatment of the globin T2 and alkaline phosphatase-resistant fragments withpenicillium P1 nuclease and alkaline phosphatase yielded a P1-resistant core structure in both fragments. In addition to the core, 2'-O-methylcytidine (Cm) was released from the more negatively charged globin fragment. The P1-resistant cores of the cap structures eluted from DEAE-Sephadex with the known standard m2G5'ppp5'Am and were found to be pyrophosphatase-sensitive establishing a 5'-5'-triphosphate linkage. The pyrophosphatase and alkaline phosphatase digestion products of the globin Cap 1 and Cap 2 core structures were analyzed by high voltage electrophoresis and paper chromatography and found to be 7-methyiguanosine (m7G) and the dimethylated nucleoside 6-methyl-2'-O-methyladenosine (N6mAm). A small amount of the singularly methylated adenosine, 2'-O-methyladenosine (Am) was also observed. The predominant sequences of the methylated nucleosides in the globin cap structures are therefore m7G5'ppp5'N6mAm and m7G5'ppp5'N6mAmpCm.