Low-molecular-weight peptide inhibits RNA synthesis in human leukemic and phytohemagglutinin-stimulated leukocytes and globin mRNA transcription in differentiating Friend cells.

The RNA synthesis of human leukemic leukocytes and phytohemagglutinin-stimulated lymphocytes is markedly reduced by administration of a low-molecular-weight nonhistone peptide factor from calf thymus. Treatment with the factor strongly inhibits hemoglobin production and globin mRNA transcription in dimethyl sulfoxide-stimulated Friend cells without appreciably modifying the rate of cell growth. Evidence for specificity of these effects is provided by the lack of action of the factor on both growth rate and RNA synthesis of a number of nondifferentiating cell lines from various animal species. After removal of the compound, both human lymphocytes and Friend cells can be stimulated by phytohemagglutinin and by dimethyl sulfoxide, respectively, ruling out any toxic effect.