Hyperacetylation of histone H4 promotes chromatin decondensation prior to histone replacement by protamines during spermatogenesis in rainbow trout.

During the final stages of spermatogenesis in rainbow trout a dramatic increase in the level of histone H4 hyperacetylation is observed which is closely correlated with the replacement of histones by protamines. In order to understand further how H4 hyperacetylation might assist in protamine replacement of the histones, we have investigated the effect of H4 hyperacetylation on chromatin structure in trout testes actively undergoing the replacement process. Long chromatin fragments enriched in hyperacetylated H4 have been isolated and characterized. Evidence is presented that hyperacetylated H4 is clustered in certain regions (domains) of late stage testis chromatin and within these domains the chromatin exhibits an altered, highly relaxed structure which is believed to be the result of the extensive hyperacetylation. These domains, which are nearly devoid of protamine, are postulated to represent an initial structural transition which is necessary for the proper histone removal and protamine replacement process to take place.