Chromatin remodeling initiates transcriptional and post-transcriptional silencing inNeurospora

Abstract

ABSTRACTIn many eukaryotes, including mammals, transcriptional (heterochromatin-mediated) and post-transcriptional (RNAi-mediated) silencing of repetitive DNA can be initiated by poorly understood mechanisms independent of strong sequence-specific cues. We found that in the fungusNeurospora crassaboth types of silencing were induced by the aberrant binding of transcription factors to a synthetic repetitive locus represented by thetetOoperator array. Transcriptional silencing of the array was mediated by canonical constitutive heterochromatin. On the other hand, post-transcriptional silencing resembled quelling, but occurred normally in the absence of RAD51 and RAD52, two principal recombination factors. Therefore, this process was named recombination-independent quelling (RIQ). Initiation of RIQ and heterochromatin was associated with perturbed chromatin, and both processes required SAD-6, a SWI/SNF chromatin remodeler orthologous to ATRX. Without SAD-6, and while still occupied by transcription factors, the locus featured an extreme state that was associated with strong nucleosome depletion and high MNase sensitivity, however, this state was unable to promote silencing. We also found that native AT-rich DNA (which comprises relics of transposable elements inN. crassa) could undergo RIQ according to the same principles. These results suggested a general model in which thede novosilencing can be induced by the active remodeling of a perturbed (nucleosome-depleted) chromatin state.