The effect of x-radiation on some components of the nuclear and cytoplasmic machinery for protein synthesis was investigated. The incorporation of14 C-leucine and <tex-math>${}^{14}{\rm C}\text{-lysine}$</tex-math> into nuclear and cytoplasmic proteins of irradiated and nonirradiated regenerating or normal rat liver was investigated. X-radiation interferes with the incorporation of <tex-math>${}^{14}{\rm C}\text{-lysine}$</tex-math> (but not with14 C-leucine) into nuclear proteins of regenerating liver. X-radiation also alters the incorporation of <tex-math>${}^{14}{\rm C}\text{-lysine}$</tex-math> into isolated nuclei obtained from regenerating liver of irradiated animals. Microsomes and ribosomes obtained from regenerating livers of irradiated or nonirradiated animals were equally efficient in contributing to amino acid incorporation into protein in standard incubation mixtures. Nuclear RNA extracted with hot phenol from irradiated or nonirradiated regenerating liver, stimulates amino acid incorporation in vitro to the same degree whatever its source. The results suggest that the alteration in protein synthesis produced by x-radiation is caused by a decrease in the production of nuclear RNA and possibly, specific types of messenger RNA.

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