Studies have been made on the incorporation of [8H]-deoxythymidine into the DNA of the livers of Rana catesbeiana tadpoles. When metamorphosis was induced with tri-iodothyronine, the specific activity of the nuclear DNA rose 5 days after administration of the hormone. In contrast, the specific activity of the DNA from the mitochondrial fraction rose grave–2 days after hormone administration.
In order to determine whether the in vivo change was due to alterations in the pool sizes of the DNA precursors, in vitro studies on DNA polymerase were carried out. It was found that under conditions where the enzyme activity was not limited by availability of template or substrates, there was a rise in the DNA polymerase activity in crude cell extracts from the tadpole liver. Fractionation of the cell components showed that little of this increment in activity appeared to be located in the nucleus, but that a large percentage alteration in activity occurred in the mitochondrial and cell sap fractions.
A possible interpretation of these results is that an increase in the mitochondrial DNA polymerase is one of the early effects of thyroid hormone. This possibility is discussed in relation to the other known effects of thyroid hormones in tadpoles, with particular reference to nucleic acid metabolism and also to mitochondrial hyperplasia.