Peptides regulate the activity of RNA polymerases in Tetrahymena

An exponentially multiplying population of the protozoa Tetrahymena has a highly variable transcription rate when cultivated in a complex broth medium. It is shown that peptides in the medium specifically stimulate transcription and that the cells in response synthesize a peptide with a molecular weight of 3000-4000. This peptide inhibits transcription in vivo in cells with high rates of transcription and in in vitro transcription systems. We have partially purified radioactively labelled inhibitor peptide and found that cells stimulated to high transcription rate selectively accumulate inhibitor peptide in the nuclei. In vitro experiments have shown that the inhibitor peptide can reduce the RNA polymerase activity in the nuclei. The presence of inhibitor seems to lower the affinity of RNA-polymerases towards DNA and to increase their release during incubation of isolated nuclei. On the other hand, stimulatory peptides will compete with the inhibitor, and the polymerase activity in isolated nuclei is determined--at least partially--by the ratio between stimulatory and inhibitory peptide factors. It is suggested that regulatory peptides also are involved in regulation of transcription in vivo.