Polysomes have been isolated from rat brain and characterized by their appearance in the electron microscope and by their sedimentation in sucrose density gradients. Rats were isolated for 3 days in the dark and were then returned to the light for 15 minutes. The polysomes in brain, but not in liver, decreased in rats deprived of light and increased in those stimulated with light. These findings together with an increased capacity for protein synthesis in the brain in vitro and in vivo suggest that an increase in the activity of messenger RNA in the brain may result from environmental changes.
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