Nuclear protein organization and the repair of radiation damage

A complex network of proteins having attachment sites with DNA are known to exist in mammalian cells and have been referred to as a nuclear cage, matrix, scaffold and nucleoid. Since ionizing radiation is known to induce DNA--protein crosslinks as well as DNA single- and double-stranded breaks, an investigation of the sedimentation of the nucleoid in Chinese hamster ovary (CHO)* cells before, during and after treatments with ionizing radiation was undertaken. Using neutral sucrose gradient sedimentation, it was possible to reproducibly separate the protein and DNA components of interphase nucleoids. Under conditions of radiation damage, the DNA and protein components of the nucleoid were shifted to a coincident position in the gradients consistent with the generation of single- and double-stranded DNA scissions. During DNA damage repair, an apparent recruitment of protein to the nucleoid occurred and a rearrangement of the protein sedimentation was observed as the repair of DNA progressed. These data suggest that the protein component of the nucleoid was dynamic under conditions of DNA damage repair.