Transition from a nucleosome-based to a protamine-based chromatin configuration during spermiogenesis in Drosophila

In higher organisms, the chromatin of sperm is organised in a highly condensed protamine-based structure. In pre-meiotic stages and shortly after meiosis, histones carry multiple modifications. Here, we focus on post-meiotic stages and show that also after meiosis, histone H3 shows a high overall methylation of K9 and K27 and we hypothesise that these modifications ensure maintenance of transcriptional silencing in the haploid genome. Furthermore, we show that histones are lost during the early canoe stage and that just before this stage, hyper-acetylation of histone H4 and mono-ubiquitylation of histone H2A occurs. We believe that these histone modifications within the histone-based chromatin architecture may lead to better access of enzymes and chromatin remodellers. This notion is supported by the presence of the architectural protein CTCF, numerous DNA breaks, SUMO, UbcD6 and high content of ubiquitin, as well as testes-specific nuclear proteasomes at this time. Moreover, we report the first transition protein-like chromosomal protein, Tpl(94D), to be found in Drosophila. We propose that Tpl(94D)--an HMG box protein--and the numerous DNA breaks facilitate chromatin unwinding as a prelude to protamine and Mst77F deposition. Finally, we show that histone modifications and removal are independent of protamine synthesis.