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

Methionine sulfoximine in vivo modifies the tRNALys pool of developing rat brain

tRNA was extracted from brains of 3-, 8-, and 18-day-old rats that were injected intracerebrally, 45 min before death, with [3H]methyl methionine or [8-3H]guanosine, and intraperitoneally, 3 h before death, with L-methionine-dl-sulfoximine (MSO), a methylation-activating convulsant agent. Although there was no effect of age or of MSO on the per gram yield of tRNA, its specific radioactivity (dpm/A260) was highest at 3 days in both the control and the MSO groups. Age- and MSO-related changes in tRNALys content of the brain tRNA pool were investigated by means of benzoylated DEAE-cellulose (BDC) and reverse-phase chromatography (RPC). BDC chromatography revealed tRNALys species in the brains of the MSO-treated animals that were absent in control brains. Of particular interest was the finding that differences in RPC-5 chromatographic mobility between control and MSO-tRNALys species were abolished by conversion of lysyl-tRNA, suggesting that the MSO-elicited change(s) in tRNALys structure involved the binding site(s) for lysine. Two additional findings were made: (a) lysine acceptance by the [3H]methyl-labeled tRNALys purified from brains of the MSO-treated animals was higher than that of controls at 18 days; and (b) omission of the BDC chromatographic step accentuated the differences in mobility on RPC-5 columns between tRNALys species of control and MSO-treated animals. Lastly, we found that some tRNALys species of control and MSO-treated brains contained significantly different proportions of N2-methyl guanine and 1-methyl adenine, relative to controls. These MSO-elicited changes in the methyl base content of tRNALys of immature rat brain are the first evidence of an alteration of brain tRNA structure by a centrally acting excitatory agent.

The biosynthesis of transfer ribonucleic acid in the developing rat brain and in cultured glial cells

The biosynthesis of tRNA was investigated in cultured astroglial cells and the 3-day-old rat brain in vivo. In the culture system astrocytes were grown for 19 days and were then exposed to [3H]guanosine for 1.5-7.5 h; 3-day-old rats were injected with [3H]guanosine and were killed 5-45 min later. [3H]tRNA was extracted, partially purified, and hydrolyzed to yield [3H]guanine and [3H]methyl guanines. The latter were separated from the former by high performance liquid chromatography and their radioactivity determined as a function of the time of exposure to [3H]guanosine. The findings indicate that labeling of astrocyte tRNA continued for 7.5 h and was maximal, relative to total RNA labeling, at 3 h, while in the immature brain tRNAs were maximally labeled at 20 min after [3H]guanosine administration. The labeling pattern of the individual methyl guanines differed considerably betweren astrocyte and brain tRNAs. Thus, [3H]1-methylguanine represented up to 35% of the total [3H]methyl guanine radioactivity in astrocyte [3H]tRNA, while it became only negligibly labeled in brain [3H]tRNA. Conversely, brain [3H]tRNA contained more [3H]N2-methylguanine than did astrocyte [3H]tRNA. Approximately equal proportions of [3H]7-methylguanine were found in the [3H]tRNAs of both neural systems. The [3H]methylguanine composition of brain [3H]tRNA was followed through several stages of tRNA purification, including benzoylated DEAE-cellulose and reverse phase chromatography (RPC-5), and differences were found between the [3H]methylguanine composition of RPC-5 fractions containing, respectively, tRNAlys and tRNAphe. The overall results of this study suggest that developing brain cells biosynthesize their particular complement of tRNAs actively and in a cell-specific manner, as attested by the significant differences in the labeling rates of their methylated guanines. The notion is advanced that cell-specific tRNA modifications may be a prerequisite for the successful synthesis of cell-specific neural proteins.

Cerebellar tRNA methyltransferases: a developmental study

Developmental patterns of homologous and heterologous tRNA methylation by cerebellar tRNA methyltransferases are described. The study revealed that: (a) homologous tRNA methylation results in the predominant formation of N2-methylguanine and 1-methyladenine; (b) tRNA methyltransferase of bulk-isolated Purkinje and granule cells methylate E. coli tRNAglu2 in vitro in a characteristic manner, and (c) the methylation of 8-day-old cerebellar, cortical and hepatic tRNA in vivo yields tRNAs containing different proportions of methylated bases. The findings suggest that the presumably cell-specific populations of cerebellar tRNA methyltransferases continue to alter their substrate recognition characteristics up to and beyond the first month of post-natal life.

Age-dependent changes in the specificity of tRNA methyltransferases in the cerebellum of the icteric and nonicteric Gunn rat

The activity of tRNA methyltransferases present in the cerebellum of 6- and 21-day-old nonicteric and icteric Gunn rats was compared using purified E. coli tRNAs as substrates. At 6 days the tRNA methyltransferases of the icteric animals were significantly more effective in methylating tRNAGlu2 and tRNAPhe than were those of their nonicteric counterparts. This relationship reversed itself at 21 days. The action of the tRNA methyltransferases from the 6-day-old icteric animals led to higher proportions of 1-methyladenine in tRNAGlu2 and tRNAPhe than were obtained using the corresponding enzymes of the nonicteric animals. The proportion of N2-methylguanine was also higher, yet only in tRNAfMet and not in tRNAPhe. The study reveals much more extensive fluctuations in the activity and in the substrate recognition specificity among the cerebellar tRNA methyltransferases of the icteric than among those of the nonicteric controls during the crucial 6--21 day period of cerebellar development.