HISTONE H4 ACETYLATION IN PLANT HETEROCHROMATIN IS ALTERED DURING THECELL-CYCLE

Using polyclonal antibodies directed against acetylated isoforms of hi stone H4 (H4 acetylated at lysine positions 5, 8, 12, 16 and H4 tetraa cetylated), indirect immunofluorescence revealed hyperacetylation for all H4 variants at the nucleolus organizer region (NOR) of metaphase c hromosomes of the field bean Vicia faba. The transcriptionally inactiv e and late-replicating heterochromatin regions proved to be hypoacetyl ated at lysine positions 5, 8 and 12. The remaining chromatin showed a verage fluorescence. These patterns were altered when deacetylase was blocked by exposure of root tip meristems to trichostatin A for more t han 2 h prior to fixation. Under these conditions, all lysine position s, except lysine 8, appeared to be hyperacetylated at the NOR and in a ddition at the prominent heterochromatin domains. This observation rep resents a hitherto unique switch of histone acetylation pattern during the cell cycle. This is apparently caused by deposition of acetylated H4.Ac5, 12 and 16 or by acetylation directly after replication, which later on becomes reduced (H4.Ac16) or even reversed (H4.Ac5 and 12) b y deacetylase before cells enter mitosis.