Tetrahymena cells secrete a low-molecular weight factor which inhibits RNA synthesis in vivo and in vitro

Medium which has been dialyzed against a dense population of Tetrahymena cells contains a factor which reduces the rate of RNA synthesis in other Tetrahymena cells inoculated on that medium. Furthermore, this extracellular factor inhibits the ribosomal RNA synthesis in vitro when added to an RNA synthesizing r-chromatin preparation.