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Schermer, Ulrike (2007): Mechanism of chromatin reassembly at the yeast PHO5 promoter upon repression. Dissertation, LMU München: Fakultät für Biologie



The goal of this study has been to elucidate the mechanisms responsible for rebuilding nucleosomes at the PHO5 promoter upon rerepression. In this work, I could unambiguously show that histones are incorporated at the PHO5 promoter upon repression. Regarding the source of these histones, I provide evidence that a significant fraction of the deposited histones originate from a soluble histone pool, i.e. a histone source in trans. Promoter closure occurs with strikingly rapid kinetics and is independent of replication. In agreement with the finding that PHO5 repression does not require cell division, I found that histone chaperones which are associated with replication-independent nucleosome assembly are important for rapid PHO5 promoter closure. Strains deleted for histone chaperones involved in replication-dependent nucleosome assembly did not exhibit any defect in promoter closure. Other factors contributing to rapid PHO5 repression turned out to be nucleosome remodelers, whose characteristic mode of action is chromatin assembly in trans. Nucleosome remodeling mutants typically catalyzing nucleosome movements in cis are not implicated in PHO5 promoter reassembly. The phenomenon of trans-deposition of histones upon repression is not restricted to the PHO5 promoter but is also found at two other phosphate regulated promoters, PHO8 and PHO84. By its rapid mode of action, this mechanism contributes to efficiently shutting off transcription. This might also hold true for other yeast genes. In the second part of this work I present results that indicate a role for the histone chaperone Asf1p in the activation of the PHO5 gene. Interestingly, the induction of PHO5 in an asf1 mutant is dependent on the phosphate concentration of the growth medium. Full induction occurs only when the medium is completely free of phosphate. The abundance of even trace amounts of phosphate precludes PHO5 activation altogether.