Helbing C C, Atkinson B G
Department of Zoology, University of Western Ontario, London, Canada.
J Biol Chem. 1994 Apr 22;269(16):11743-50.
One of many changes occurring during spontaneous and 3,5,3'-triiodothyronine (T3)-induced metamorphosis of the Rana catesbeiana tadpole is the permanent transition from an ammonotelic, aquatic larva to a ureotelic, terrestrial adult. T3-induced urea production is preceded by T3-induced elevation in the synthesis and level of liver-specific urea cycle enzymes essential for detoxication of ammonia in a terrestrial environment. This report focuses on establishing the effects heat shock (hs) has on the T3-induced expression of genes encoding three essential urea cycle enzymes. We demonstrate that hs stabilizes the intracellular existing levels of carbamyl-phosphate synthetase I (CPS I), the first enzyme in the urea cycle, while concurrently depressing its new synthesis. To establish the effects of hs on CPS I mRNA levels, we characterized cDNAs encoding an amphibian CPS I and demonstrate that it may represent an evolutionary link between microbial CPS and mammalian CPS I. Using this CPS I cDNA and other R. catesbeiana gene-specific probes, we demonstrate that hs depresses the level of T3-induced thyroid hormone receptor beta mRNAs but does not affect the level of T3-induced CPS I, ornithine transcarbamylase, and arginase mRNAs. These results support the contention that the hs response may involve the selective protection of some pre-existing mRNAs and proteins essential for an organism's survival.
在牛蛙蝌蚪自发变态以及3,5,3'-三碘甲状腺原氨酸（T3）诱导的变态过程中发生的众多变化之一，是从排氨的水生幼体到排尿素的陆生成体的永久性转变。T3诱导尿素生成之前，T3会使肝脏特异性尿素循环酶的合成和水平升高，这些酶对于在陆地环境中氨的解毒至关重要。本报告重点在于确定热休克（hs）对T3诱导的编码三种必需尿素循环酶的基因表达的影响。我们证明，热休克可稳定尿素循环中第一种酶氨甲酰磷酸合成酶I（CPS I）的细胞内现有水平，同时抑制其新合成。为了确定热休克对CPS I mRNA水平的影响，我们对编码两栖动物CPS I的cDNA进行了表征，并证明它可能代表微生物CPS和哺乳动物CPS I之间的进化联系。使用该CPS I cDNA和其他牛蛙基因特异性探针，我们证明热休克会降低T3诱导的甲状腺激素受体β mRNA的水平，但不影响T3诱导的CPS I、鸟氨酸转氨甲酰酶和精氨酸酶mRNA的水平。这些结果支持了这样的观点，即热休克反应可能涉及对生物体生存所必需的一些预先存在的mRNA和蛋白质的选择性保护。
