EVIDENCE FOR THE COMPLETE DNA-DEPENDENCE OF RNA SYNTHESIS IN ISOLATED THYMUS NUCLEI

Abstract

Ribonucleic acid (RNA) synthesis in isolated thymus nuclei which have lost most of their deoxyribonucleic acid (DNA) is directly proportional to the amount of DNA that remains. The same is true of thymidine incorporation into DNA. However, the uptake of amino acids into nuclear proteins is not directly DNA-dependent by this test. From the distribution of radioactive P in the nucleo-tides obtained from RNA it can be concluded that nearly all of the RNA synthesized in the isolated thymus nucleus is of the "messenger" type. Evidence is presented that most of the DNA in the isolated nucleus is inactive or repressed, as far as RNA synthesis is concerned. This is considered an indication of specialization of chromosomal regions in controlling the growth and development of highly differentiated tissues. The repression of DNA activity by histone proteins is presented as a means of controlling the synthesis of RNA at different sites on the chromosome.