The developmental pattern of homologous and heterologous tRNA methylation in rat brain differential effect of spermidine

Abstract

UsingS-adenosyl-L-[Me-¹⁴C] methionine, rat cerebral cortex methyltransferase activity was determined during the early postnatal period in the absence of addedEscherichia coli tRNA and in its presence. [Me-¹⁴C] tRNA was purified from both systems and its [Me-¹⁴C] base composition determined. The endogenous formation of [Me-¹⁴C] tRNA (homologous tRNA methylation) was totally abolished in the presence of 2.5 mM spermidine, whereasE. coli B tRNA methylation (heterologous methylation) was markedly stimulated. Only [Me-¹⁴C] 1-methyl guanine and [Me-¹⁴C]N ²-methyl guanine were formed by homologous methylation, there being an inverse shift in their relative proportions with age. Heterologous tRNA methylation led, additionally, to the formation of [Me-¹⁴C]N ₂ ² -dimethyl guanine, 5-methyl cytosine, 1-methyl adenine, 5-methyl uracil, 2-methyl adenine, and 1-methyl hypoxanthine. A comparison of heterologous tRNA methylation between the whole brain cortex (containing nerve and glial cells) and bulk-isolated nerve cell bodies revealed markedly lower proportions of [Me-¹⁴C]N ₂-methyl andN ₂ ² -dimethyl guanine and significantly higher proportions of [Me-¹⁴C] 1-methyl adenine in the neurons. The present findings suggest (1) that homologous tRNA methylation may provide developing brain cells with continuously changing populations of tRNA and (2) that neurons are enriched in adenine residue-specific tRNA methyltransferases that are highly sensitive to spermidine.