Initiation of RNA Synthesis in Isolated Nuclei

Ribonucleotide triphosphates, labeled at the β position, were synthesized and used directly to quantify RNA chain initiation in nuclei isolated from Friend cells grown in tissue culture. At the optimal salt concentration, low‐molecular‐weight RNAs (4–5S) synthesized by RNA polymerase III were the predominant species initiated. Less than 5% of the molecules were initiated by polymerase II. We calculate that 50–80% of the small RNA molecules synthesized in vitro were also initiated in vitro. Assuming that a substantial fraction of the nuclei were active in vitro, the number of 4–5S RNA molecules initiated per nucleus was about 100 molecules/min.