Studies on the specialized transfer RNA population of the lens

Rat lens beta-crystallins are internally duplicated and homologous to gamma-crystallins

The nucleotide sequence of two cloned rat lens beta-crystallin cDNAs pRL beta B3-2 and pRL beta B1-3 has been determined. pRL beta B3-2 contains the complete coding information for a beta-crystallin, designated beta B3, of 210 amino acid residues. pRL beta B1-3 is incomplete at its 5' end; the 5' codogenic information which is not present in this cDNA clone was deduced from the cloned gene. pRL beta B1-3 codes for a beta-crystallin polypeptide, designated beta B1, whose full length is 247 amino acid residues. Considerable sequence homology is noted between the amino- and carboxy-terminal halves of each protein. The two rat beta-crystallins show a substantial sequence homology with each other (60%) as well as with the published sequences of rat gamma-crystallin (37%) and bovine and murine beta-crystallins (55 and 45%). All these proteins have a two-domain structure which, like the bovine gamma II-crystallin, might be folded into four remarkably similar protein motifs. Our data further indicate that the beta-crystallins can be subdivided into two groups which are evolutionarily related. Both groups are, although more distantly, also related to the gamma-crystallins.

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.

Alterations in tRNA isoaccepting species during erythroid differentiation of the Friend leukemia cell

The chromatographic profiles of isoaccepting tRNAs were analyzed at five time points during the 96 hr, dimethylsulfoxide induced, erythroid-like differentiation of Friend leukemia cells. Sixty-four isoaccepting species of tRNA for 16 amino acids were resolved by RPC-5 chromatography. The relative amounts of tRNAphe, tRNAile, and tRNAval species were maintained by the cells during differentiation; whereas the relative amounts of some of the isoacceptor tRNAs for the other 13 amino acids changed significantly. Fluctuations in amounts of isoacceptors occurred between 36 and 72 hr after addition of dimethysulfoxide, corresponding to globin mRNA appearance and hemoglobin synthesis, respectively. In most cases, thepredominant tRNA isoacceptors of uninduced cells were retained throughout differentiation. Notable exceptions were tRNA species for threonine, proline, and methionine. Some of the isoacceptors occurring in relatively smaller amounts were not expressed at all times. These changes possibly reflect the cell's functional adaptation of tRNA in differentiation for hemoglobin synthesis.