CAPACITIES FOR BINDING AMINO ACIDS BY tRNAs FROM RAT BRAIN AND THEIR CHANGES DURING DEVELOPMENT

–The total tRNA and some specific tRNAs from the 100,000g soluble fraction of rat brain were measured during development (postnatal ages 4–55 days). For determination of specific tRNAs we developed a method that measured their capacities to bind specific amino acids. Levels of total tRNA were decreased in the soluble fraction from the brains of 55-day-old rats in comparison to those for the 4-day-old rats. The aminoacylation capacities of tRNAs for phenylalanine, lysine, proline, valine, leucine, alanine and isoleucine were diminished in the 55-day-old rats in comparison to those for 4-day-old rats when expressed per unit wet weight of brain. When the 4- to 55-day changes in aminoacylation capacity of each specific tRNA was expressed relative to that of the total tRNA, tRNA^Phe and tRNALys^Lys were diminished; tRNA^Pro, tRNA^Vel, tRNA^GIY and tRNA^Leu showed no significant changes; and tRNA^A1a and tRNA^Ile were increased. Incorporation of amino acids into a material insoluble in hot TCA (probably proteins) in a ribosome-free system occurred in the brain preparations. Out of ten different amino acids studied, arginine and tyrosine exhibited the highest values for this type of transfer.