Replacement of protamine by F1 histone during reactivation of fused human sperm nuclei

Rabbit antisera, specific for the histones F1, F2a2, F2b, F3 and for protamine were used to monitor a possible transition from protamine towards somatic-type histones during sperm nucleus reactivation, following human sperm fusion with mouse fibroblasts. Mature human sperm nuclei were shown to contain the histones F2a2, F2b, F3 and protamine, but were missing F1 histone by immuno cytochemistry using the indirect fluorescence method. However, a gradual disappearance of protamine from fused sperm nuclei, could be observed during the first 24 h of reactivation. Subsequently, F1 histone could be detected in increasing concentrations in 60% of reactivated sperm nuclei, during the next four days. The shift from protamine towards F1 histone could also be visualized cytochemically via staining with brilliant sulphaflavine, which appears to discriminate between sperm nuclei on the basis of their F1 histone content.