Mechanism of chromatin reassembly at the yeast PHO5 promoter upon repression

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.