Effect of human placental ribonuclease inhibitor in cell-free ribosomal RNA synthesis

Abstract Exposure to purified placental ribonuclease inhibitor caused a marked reduction in ribonuclease activity associated with intact nucleoli. Addition of the placental RNase inhibitor to a nucleolar RNA synthesizing system, prior to the addition of ribonucleoside triphosphates, resulted in a significant increase in the size but did not alter the yield of the RNA products. The effect of the RNase inhibitor on the size of distribution of in , vitro transcriptional products could be reversed by the addition of N-ethylmaleimide after transcription was terminated. The properties of the ribonnclease(s) affected by the inhibitor were indicated by its resistance to N-ethylmaleimide and EDTA inhibition and by its limited endonucleolytic cleavage of the in , vitro transcript.