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Organ-specific Differences in the Methylation of Transfer RNA in Vitro¹

Department of Biochemistry, School of Dental Medicine, University of Pennsylvania, Philadelphia, Pennsylvania 19104

Tissue homogenates from tumors have been reported to differ from nonneoplastic tissue in their capacity to methylate transfer RNA (tRNA). We have compared the extent of methylation obtained with several rat organs in order to determine whether similar differences can be observed among nontumorous tissues and whether differences can be attributed to variations in tissue-specific methylating enzymes.

Enzyme preparations from rat liver and spleen show differing capacities for forming several methylated nucleosides. When assayed in the presence of optimal concentrations of polyamines, these differences are observed with both unfractionated methyl-deficient Escherichia coli tRNA and E. coli formylmethionine tRNA as substrate. Distinctions between liver and spleen are also seen when polyamines are omitted from the reaction mixture; in contrast to preparations from liver, ethylenediaminetetraacetate-treated ammonium sulfate fractions of spleen homogenates methylate tRNA in the absence of added cations. The product of this methylation is 1-methyladenosine. Differences between liver and spleen preparations disappear when the enzyme preparations are eluted from a diethylaminoethyl Sephadex column. Neither ammonium sulfate fractions nor the enzymes eluting from diethylaminoethyl Sephadex are capable of completely methylating available sites on formylmethionine tRNA under conditions designed to produce maximal extent of methylation. These results suggest that organ-specific differences observed with unpurified methyltransferases are a result of inhibitory material in the enzyme preparation.

¹ Supported by National Cancer Institute Grant CA 10586.

² Recipient of Career Development Award GM 09805.

Received 3/28/73. Accepted 6/ 8/73.