No abstract

The association between histone acetylation and replacement observed during spermatogenesis prompted us to consider the testis as a source for potential factors capable of remodelling acetylated chromatin. A systematic search of data banks for open reading frames encoding testis-specific bromodomain-containing proteins focused our attention on BRDT, a testis-specific protein of unknown function containing two bromodomains. BRDT specifically binds hyperacetylated histone H4 tail depending on the integrity of both bromodomains. Moreover, in somatic cells, the ectopic expression of BRDT triggered a dramatic reorganization of the chromatin only after induction of histone hyperacetylation by trichostatin A (TSA). We then defined critical domains of BRDT involved in its activity. Both bromodomains of BRDT, as well as flanking regions, were found indispensable for its histone acetylation-dependent remodelling activity. Interestingly, we also observed that recombinant BRDT was capable of inducing reorganization of the chromatin of isolated nuclei in vitro only when the nuclei were from TSA-treated cells. This assay also allowed us to show that the action of BRDT was ATP independent, suggesting a structural role for the protein in the remodelling of acetylated chromatin. This is the first demonstration of a large-scale reorganization of acetylated chromatin induced by a specific factor.The histone code hypothesis postulating a role for posttranslational modifications of histones in the control of chromatin structure and function (39, 41) is now supported by a large body of data. The milestone of this hypothesis is the existence of protein domains capable of specifically recognizing particular posttranslational histone modifications (22). To date, at least two distinct domains have been shown to interact with modified histones: bromodomains, recognizing acetylated lysines (43), and some chromodomains, interacting with methylated lysine 9 present in the histone H3 N-terminal tail (2,19,20,25,40).Bromodomains are conserved modules present in many chromatin-and transcription-related proteins, including histone acetyltransferases and chromatin remodelling factors (11). The first bromodomain shown to specifically interact with an acetylated peptide, corresponding to the histone H4 N terminus, was that of P/CAF (9). Further investigations showed that bromodomains can also recognize acetylated lysines in nonhistone proteins and play an essential role in establishing new acetylation-dependent functions (10, 33, 37). Bromodomain-containing proteins are therefore likely to interpret signals generated by protein acetylation and, more specifically in the case of chromatin, mediate functions depending on histone acetylation (1,16,26,31).Interestingly, there is at least one physiological situation where histone acetylation can unambiguously be linked to chromatin remodelling. In many species, including the fly, trout, rooster, rat, mouse, and human, male germ cell nuclear maturation is characterized by the hyperacetylation of histones ...