Isolation of a High Specific Activity 35S Ribosomal RNA Precursor from Tetrahymena pyriformis and Identification of Its 5' Terminus, pppAp

RNA isolated from starved and refed Tetrahymena pyriformis GL, pulse labeled with H332PO4, contains an RNA molecule of 2.4 × 10⁶ molecular weight which can be identified by polyacrylamide gel electrophoresis. This RNA has been shown to be a precursor of the mature 25S rRNA and 17S rRNA by a kinetic analysis of its synthesis, by hybridization competition with pure 25S rRNA and 17S rRNA, and by two-dimensional fingerprint analysis. The 5'-terminal nu-cleotide was isolated from the total digestion products of the 35S rRNA precursor and shown to be pppAp by analysis of the products of snake venom phosphodiesterase digestion. Low, but significant yields of the 5' terminus pppAp were also isolated from the 17S rRNA demonstrating that the primary transcript of the rRNA gene in T. pyriformis contains the 17S rRNA at its 5'end.