Histone Exchange is Associated with Activator Function at Transcribed Promoters and with Repression at Histone Loci

Transcription in eukaryotes involves major chromatin changes including the replacement of old nucleosomal histones by new histones at the promoters of genes. The role of these histone exchange events in transcription remains unclear. In particular, it is unknown if histone exchange is triggered by activators or merely a passive consequence of preinitiation complex (PIC) assembly and/or subsequent transcription. Here we show that, in yeast, histone exchange largely persists genome-wide in the absence of the PIC. The same is true at inducible promoters, where histone exchange occurs only under inducing conditions and requires the presence of the activators but not the PIC. Additionally, we provide evidence that the HIR complex silences histone gene transcription outside of S phase by continuously incorporating new histones to maintain a repressive nucleosomal environment. Thus, histone exchange is associated both with activator-function at transcribed genes and with transcriptional repression at the histone loci.