Biochemical Aspects of Metamorphosis: Changes in RNA Associated with the Transition from Ammonotelism to Ureotelism

Abstract

The relationship of RNA metabolism to the induction ot the enzyme carbamyl phosphate synthetase (the initial enzymatic step in biosynthesis of urea) in the liver of premetamorphic tadpoles exposed to thyroxine is reviewed. Treatment with thyroxine stimulates the synthesis of ribosomal, soluble, and “messenger” RNA fractions; the latter fraction shows the most rapid rate of synthesis in response to thyroxine. Properties of the “messenger” RNA fraction are reviewed. In keeping with the stimulation and formation of a “messenger” RNA fraction by thyroxine is the finding that thyroxine increases the efficiency of chromatin template for the synthesis of RNA. Although the present data stress the action of thyroxine on the efficiency of transcription, reference is made to the stimulatory effect of thyroxine at the level of translation in liver preparations from tadpoles. Thus, thyroxine has an effect at the levels of both transcription and translation.