The measurement of the production of tRNA(iMet) during erythroid differentiation of the Friend erythroleukemia cell.

The production of tRNA(iMet) during Friend cell erythroid differentiation has been studied. In vitro measurements of total nuclear RNA synthesis in nuclei isolated from Friend cells at different stages of differentiation show the total RNA synthesis increases 1.5-fold at day 1 of induction and then decreases through days 2 and 3 to approximately 75% of its rate of synthesis in the nuclei of uninduced cells. The synthesis of RNA polymerase III transcripts undergoes a similar fluctuation through day 2 of induction, but increases again at day 3. The specific synthesis of tRNA(iMet) was measured by hybridization of labelled nuclear RNA to a tRNA(iMet) gene probe. During erythroid differentiation the percentage of nuclear RNA represented by tRNA(iMet) remains constant (0.065%), so that the absolute synthesis of tRNA(iMet) fluctuates during differentiation, in the fluctuations in the synthesis of total nuclear RNA. The relative synthesis of tRNA(iMet) in vivo was studied by labelling cells with 32Pi, isolating the resulting radioactive tRNA--5S RNA population, and hybridizing this population to a tRNA(iMet) gene probe. The ratio of tRNA(iMet)/total tRNA--5S RNA in newly synthesized cytoplasmic RNA remains similar throughout differentiation (averaging 0.0171), implying that the fluctuations observed in the nuclear synthesis of tRNA(iMet) during differentiation probably also occur for the nuclear synthesis of most tRNA and 5S RNA species. Attempts were made to measure the relative steady-state concentration of tRNA(iMet) using both aminoacylation and in vitro end labelling of tRNA followed by hybridization to a tRNA(iMet) gene probe. These two methods gave different results and we discuss the possible pitfalls of using enzymatic methods for quantitating tRNA concentrations in the cell.