Sites of methylation of purified transfer ribonucleic acid preparations by enzymes from normal tissues and from tumours induced by dimethylnitrosamine and 1,2-dimethylhydrazine

摘要

p1. The sites within the tRNA sequence of nucleosides methylated by the action of enzymes from mouse colon, rat kidney and tumours of these tissues acting on tRNAsupAsp/sup from yeast and on tRNAsupGlu/supsub2/sub, tRNAsupfMet/sup and tRNAsupVal/supsub1/sub from iEscherichia coli/i were determined. 2. The same sites in a particular tRNA were methylated by all of these extracts. Thus tRNAsupGlu/supsub2/sub was methylated at the cytidine residue at position 48 and the adenosine residue at position 58 from the 5′-end of the molecule; tRNAsupAsp/sup was methylated at the guanosine residue at position 26 from the 5′-end of the molecule; tRNAsupfMet/sup was methylated at the guanosine residues 9 and 27, the cytidine residue 49 and the adenosine residue 59 from the 5′-end; tRNAsupVal/supsub1/sub was methylated at the guanosine residue 10, the cytidine residue 48 and the adenosine residue 58 from the 5′-end. 3. All of these sites within the clover leaf structure of the tRNA sequence are occupied by a methylated nucleoside in some tRNA species of known sequence. It is concluded that methylation of tRNA from micro-organisms by enzymes from mammalian tissues iin vitro/i probably does accurately represent the specificity of these enzymes iin vivo/i. However, there was no evidence that the tumour extracts, which had considerably greater tRNA methylase activity than the normal tissues, had methylases with altered specificity capable of methylating sites not methylated in the normal tissues./p