Analysis of Messenger RNA Coding for S100 Protein in the Mammalian Brain

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Abstract
S100 protein is a brain-specific protein which is absent at birth and first appears in rabbit brain 2-3 days after birth. To determine how the synthesis of this brain-specific protein is regulated, mRNA was isolated from brain polysomes and assayed for S100 protein mRNA activity by in vitro translation in a heterologous cell-free system and immunoprecipitation of released polypeptides with rabbit anti-S100 protein antiserum. S100 protein mRNA was detected primarily in small polysomes containing 5-8 ribosomes and virtually no S100 protein mRNA was present in polysomes containing more than 8 ribosomes. S100 protein mRNA was not detected in brain polysomes at stages prior to the induction of synthesis of S100 protein, i.e., in fetal brain or in 1 day neonates. The amount of S100 protein mRNA in polysomes of the cerebral cortex and cerebellum was measured to see if it correlated with the level of S100 protein in the 2 regions of adult brain. The cerebellum, which contained 3-4 times the level of S100 protein in the cerebral cortex, contained 4 times more S100 protein mRNA.