Studies on Ribonucleic Acid Synthesis in Tadpole Liver during Metamorphosis Induced by Thyroxine

Abstract

Abstract A method of preparing ribonucleic acid from tadpole liver has been described. It was found that tadpole liver RNA is unstable during the isolation procedure. Various factors, including temperature, the concentration of Mg++ in the homogenizing medium, and the presence of ribonuclease inhibitors, were found to be of importance in protecting RNA during its preparation. Three peaks with sedimentation coefficients of 29 S, 18 S, and 6 S were obtained by zonal sucrose density gradient centrifugation. The 6 S RNA was found in the purified ribosomes from mitochondrial and postmitochondrial fractions. The base ratios of 6 S RNA were different from those of the 18 S and 29 S fractions and the bulk RNA, showing in particular a low concentration of UMP. Some properties of RNase from tadpole liver have been described and discussed in relation to the base composition of 6 S RNA. The most rapidly labeled RNA appeared in the transfer RNA fraction when orotic-acid-14C was administered to untreated tadpoles. The RNA heavier than transfer RNA has the highest specific radioactivity 2 days after thyroxine treatment. The base composition of this RNA was analyzed with the use of 32Pi and found to be high in uridine 5'-phosphate (25 to 35%) in spite of the low value of the UMP in bulk RNA in this area. Rapidly labeled RNA which appears in the 6 to 10 S area has the properties of messenger RNA needed for induction of the enzymes in liver associated with metamorphosis.