Immunofluorescent and autoradiographic analysis of the relationship between the presence of histone H5 and DNA synthesis in developing chick erythroid cells

Simultaneous detection of histone H5 by indirect immunofluorescence and of [³H]thymidine incorporation by autoradiography on the same preparations of developing erythroid cells have been used to precisely define the extent of correlation between the loss of nuclear activity and the presence of histone H5. It was found that from day 3-12 of embryonic life there are two successive waves of double-labelled cells. At some stages, as many as 30% of the cells which incorporate [³H]thymidine also contain histone H5. Thus, the simple presence of H5 cannot be sufficient to cause nuclear inactivation. A kinetic analysis of the appearance and disappearance of [³H]thymidine-labelled cells, containing histone H5, and cells which are positive for both markers is presented. The result is consistent with the interpretation that the appearance of H5 in the first wave of double labelled cells occurs just before the erythroid cells become metabolically inactive. These observations modify the concept that histone H5 functions uniquely or solely as a template repressor.