3,5,3'-TRIIODOTHYRONINE-INDUCED CARBAMYL-PHOSPHATE SYNTHETASE GENE-EXPRESSION IS STABILIZED IN THE LIVER OF RANA-CATESBEIANA TADPOLES DURING HEAT-SHOCK

One of many changes occurring during spontaneous and 3,5,3'-triiodothy ronine (T-3) induced metamorphosis of the Rana catesbeiana tadpole is the permanent transition from an ammonotelic, aquatic larva to a ureot elic, terrestrial adult. T-3-induced urea production is preceded by T- 3-induced elevation in the synthesis and level of liver-specific urea cycle enzymes essential for detoxication of ammonia in a terrestrial e nvironment. This report focuses on establishing the effects heat shock (hs) has on the T-3-induced expression of genes encoding three essent ial urea cycle enzymes. We demonstrate that hs stabilizes the intracel lular existing levels of carbamyl-phosphate synthetase I(CPS I), the f irst enzyme in the urea cycle, while concurrently depressing its new s ynthesis. To establish the effects of hs on CPS I mRNA levels, we char acterized cDNAs encoding an amphibian CPS I and demonstrate that it ma y represent an evolutionary link between microbial CPS and mammalian C PS I. Using this CPS I cDNA and other R. catesbeiana gene specific pro bes, we demonstrate that hs depresses the level of T-3-induced thyroid hormone receptor beta mRNAs but does not affect the level of T-3-indu ced CPS I, ornithine transcarbamylase, and arginase mRNAs. These resul ts support the contention that the hs response may involve the selecti ve protection of some pre-existing mRNAs and proteins essential for an organism's survival.