Chicken erythrocyte histone H5 has been suggested repeatedly to be a general suppressor of transcription and replication. Therefore, the biological functions of H5 were investigated and compared with those of H1 (H1a + H1b) by microinjection of the purified proteins into proliferating L6 rat myoblasts. By pulse-labelling of the injected cells with [3H]uridine and [3H]thymidine it was shown that H5 blocked both transcription and replication substantially, and that the chromatin of the injected cells became densely compacted. H1 also suppressed these functions, but to a much lesser degree. The effects were specific and not caused by change in intracellular pH caused by introduction of the very basic H5, or its non-specific interaction with nucleic acid, since injection of protamine or lysozyme did not affect the cells. The migration and localization of injected H5 was monitored at different times after injection by immunofluorescence, which revealed that H5 was efficiently and stably concentrated in the nucleus. The results indicate that H5 indeed might function as an inactivator of the erythroid genome in its natural environment, probably by keeping the chromatin in a very condensed state.

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