Supercoiled DNA repair in thymocyte fractions differing in radiosensitivity.

Mouse thymocyte fractions were isolated on the basis of buoyant density in a step-gradient of human serum albumin. These fractions were characterized by cell size, radiosensitivity and hydrocortisone sensitivity, and by labelled precursor incorporation into DNA, RNA and proteins. The relative sedimentation of nucleoids in sucrose gradient was also determined. Large and more radioresistant thymocytes were characterized by an increased rate of DNA repair compared with the fraction of radiosensitive small lymphocytes. Nucleoids prepared from the small thymocyte fraction had greater relative sedimentation rates than those derived from large cells. The response of relative sedimentation of nucleoids from large and small thymocyte fractions to ethidium bromide concentration does not permit a conclusion on different superhelix density for DNA in cells of these fractions. At the same time the estimate of the supercoiled domain size for nucleoids of large and small thymocyte fractions showed that in the more radiosensitive small cells domains of greater size were predominant.