Amino acids and control of nucleolar size, the activity of RNA polymerase I, and DNA synthesis in liver.


The volume of nucleolar material per nucleus and the activity of RNA polymerase I (RNA nucleotidyltransferase I) become doubled in the liver cells of rats that are fed for several days a diet that lacks essential amino acids. Omission of methionine from a fully supplemented diet is equivalent to leaving out all the amino acids, and the responses to a deficiency of tryptophan are about 40% as great. Deprivation of one of the remaining essential amino acids gives either small responses or none at all. Supplementation of the methionine-free diet with cystine blocks the nucleolar enlargement and the enhancement of the polymerase activity that would otherwise take place, but the dispensable amino acid does not affect the responses to a deprivation of one of the other essential amino acids. After deprivation of all the essential amino acids or only methionine, hepatocytes make DNA when the rat is fed a meal with protein. A preparatory diet lacking in tryptophan is much less effective; a deficiency in any of the other indispensable compounds tested fails to prepare the liver for DNA synthesis. The results give hope that elucidation of the means by which methionine deprivation affects the nucleolus will also provide information on the regulation of nuclear DNA replication in liver. One attractive possibility is that the amino acid deficiency acts by producing some imbalance in protein metabolism.

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