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L-Arginine-glycine amidinotransferase (transamidinase) is the first and rate-limiting step in creatine biosynthesis. Rats fed a creatine-supplemented diet or hypophysectomized rats have only 20% of the kidney transamidinase activity as intact rats fed a creatine-free diet. A cDNA clone corresponding to transamidinase was isolated by immunoscreening of a lambda gt11 expression library prepared from rat kidney mRNA. The transamidinase cDNA had an open reading frame containing the known sequence of the amino-terminal peptide of transamidinase. Based on the cDNA sequence, transamidinase is synthesized as a precursor with an amino-terminal extension of 50 amino acids, consistent with its mitochondrial localization. Comparison of the transamidinase sequence with the protein data base identified only a single, related protein. Remarkably, this protein, which has a 37% amino acid identity with transamidinase, is also an amidinotransferase, catalyzing streptomycin biosynthesis in Streptomyces griseus. Transamidinase cDNA was used to investigate the regulation of mRNA levels by creatine and growth hormone. Hypophysectomized rats were fed a creatine-free or a creatine-supplemented diet and maintained with and without injections of growth hormone. An excellent correlation was found between changes in transamidinase activity and mRNA levels in response to creatine and growth hormone. Thus, the regulation of transamidinase by creatine and growth hormone is at a pretranslational level. In addition, the two effectors do not act independently but interact at a pretranslational level to control transamidinase gene expression. |
