N2-guanine specific transfer RNA methyltransferase II from rat liver |
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Authors: | Kraus J Staehelin M |
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Affiliation: | Biological Laboratories, Pharmaceutical Department, CIBA-GEIGY Ltd., Basel, Switzerland |
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Abstract: | An enzyme was purified from rat liver and leukemic rat spleen which methylates guanosine residues in tRNA to N2-methylguanosine. By sequence analysis of bulk E. coli tRNA methylated with crude extracts it was shown that the enzyme is responsible for about 50% of total m2G formed invitro. The extent of methylation of a number of homogenous tRNA species was measured using the purified enzyme from both sources. Among tested E. coli tRNAs only tRNAArg, tRNAPhe, and tRNAVal yielded significantly more m2G than the bulk tRNA. The Km for tRNAArg in the methylation reaction with enzymes from either tissue was 7.8 × 10−7 M as compared to the value 1 × 10−5 M obtained for the bulk tRNA. In a pancreatic RNase digest of bulk tRNA as well as of pure tRNAArg, tRNAPhe, and tRNAVal, A-m2G-Cp was found to be the only sequence methylated. Thus, the mammalian methyltransferase specifically recognizes the guanylate residue at position 10 from the 5′-end contained in a sequence (s4)U-A-G-Cp. Furthermore, there is no change between the enzyme from normal liver and leukemic spleen in the affinity for tRNA, the methylating capacity, and tRNA site and sequence recognition specificity. |
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