Regulation of protein synthesis during postnatal maturation of mouse brain.

On a DNA basis, there is higher concentration of polysomes in the brain of newborn than in the brain of adult mice, but there is no maturation-dependent decrease in tRNA content during postnatal development. The amino acid incorporating activity of cell-free systems with polysomes or mitochondria from newborn brain exceeds that of adult controls significantly in contrast to a smaller incorporating rate of labelled amino acids into synaptosomal protein. Addition of polysomes isolated from newborn brain increases the amino acid incorporation by cell-free systems with adult brain tRNA and enzymes, whereas the polysomes from adult brain decrease the incorporating activity of newborn brain systems. The loading capacity of newborn brain tRNA exceeds that of the adult controls and the velocities of its aminoacylation are four times faster. Uncharged as well as precharged newborn brain tRNA increases the amino acid incorporating activity of tRNA-dependent cell-free systems with adult brain polysomes and enzymes. In contrast to polysomes and tRNA, the newborn brain enzymes involved in protein synthesis seem to be less active in cell-free amino acid incorporation than the enzyme fractions from adult brain. These data indicate that the different protein synthesizing activity in developing and adult mouse brain is the result not only of higher amino acid incorporating activities of the newborn polysomes, but also of a stimulated acceptance and transfer function of the newborn brain tRNA.