Characterization of isoaccepting transfer RNA species changes during friend cell erythroid differentiation
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Erythroid differentiation in Friend leukemia cells can be induced by addition of dimethylsulfoxide to the culture medium. Hemoglobin represents 24% of the protein synthesis in differentiated cells. Therefore, Friend cells induced for differentiation may exhibit a marked change in tRNA population in order to effectively translate globin mRNA. Friend cells may prove to be a model system for the study of change in tRNA expression during differentiation and for the facilitation of specific protein synthesis. The chromatographic profiles of isoaccepting tRNA species from Friend leukemia cells were analyzed at five time points during a ninety-six hour period of dimethyl sulfoxide-induced differentiation. Sixty-four isoaccepting species of tRNA for sixteen amino acids were resolved by RPC-5 chromatography. The relative amounts of tRNA^phe, tRNA^ile, and tRNA^val species were maintained by the cells during differentiation; whereas the relative amounts of some of the isoaccepting tRNAs for the other thirteen amino acids changed significantly. Transfer RNA species containing the hypermodified nucleoside Q were among those exhibiting significant changes. Fluctuations in relative amounts of isoacceptors occurred between 36 and 72 hr after addition of dimethyl sulfoxide, corresponding to globin mRNA appearance and hemoglobin synthesis, respectively. In most cases, the predominant tRNA isoacceptors of uninduced cells were retained throughout differentiation. Notable exceptions were tRNA species for threonine, proline, and methionine. The two proline tRNA species present in uninduced cells were replaced by two different species in induced cells; the four threonine isoacceptors present early were represented by only two of the four in differentiated cells. These changes may reflect the cell's response to globin mRNA containing only two of the four threonine codons. Initiator methionine tRNA decreases in relative amount during the first 48 hours and then increases. This decrease is correlated to an initial lag in cell growth and protein synthesis. 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. The amount and distribution of Q-base-containing isoacceptors of tRNA^asn, tRNA^asp, tRNA^his, and tRNA^tyr were assayed. The amount of Q-base-containing tRNA species decreased in the first 48 hr after the induction, then increased again, indicating the level of Q modification is correlated to the process of differentiation.
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