DIFFERENCES OF HISTONE-H4 ACETYLATION AND REPLICATION TIMING BETWEEN A-CHROMOSOMES AND B-CHROMOSOMES OF BRACHYCOME-DICHROMOSOMATICA

Differences are demonstrated between A (transcriptionally active) and B (transcriptionally inactive) chromosomes that are characterized by a different level of histone H4 acetylation and a different timing of D NA replication. These differences between the chromatin of A and B chr omosomes were found after immunolabelling of chromsomes of Brachycome Dichromosomatica with antibodies specific for different acetylated for ms (lysine 5, 8, 12 and 16) of histone H4. In contrast to the A chromo somes, which are labelled brightly in their entirety, the transcriptio nally inactive B chromosomes are faintly labelled with antibodies agai nst H4Ac5 and H4Ac8. No such difference between the chromosomes is fou nd after immunostaining with the other antibodies H4Ac12 and H4Ac16. A nalysis of DNA replication timing in root-tip meristems suggests that B chromosomes are labelled late in S-phase compared with A chromosomes . After C-banding the B chromosome appeared to have a similar amount o f heterochromatin to the A chromosomes.