Regulation of Heterochromatic Silencing and Histone H3 Lysine-9 Methylation by RNAi

Abstract

Eukaryotic heterochromatin is characterized by a high density of repeats and transposons, as well as by modified histones, and influences both gene expression and chromosome segregation. In the fission yeast Schizosaccharomyces pombe, we deleted the argonaute, dicer, and RNA-dependent RNA polymerase gene homologs, which encode part of the machinery responsible for RNA interference (RNAi). Deletion results in the aberrant accumulation of complementary transcripts from centromeric heterochromatic repeats. This is accompanied by transcriptional de-repression of transgenes integrated at the centromere, loss of histone H3 lysine-9 methylation, and impairment of centromere function. We propose that double-stranded RNA arising from centromeric repeats targets formation and maintenance of heterochromatin through RNAi.

Chromatin structural changes around satellite repeats on the female sex chromosome in Schistosoma mansoni and their possible role in sex chromosome emergence

“…The transcription of repetitive elements of the satellite type in S. mansoni is particularly interesting in the light of the recent discovery of stage-dependent expression of the elements that constitute the RNA interference (RNAi) pathway in schistosomes [39,40]. In many organisms RNAi and chromatin structural changes are linked [32,41-43] and it is tempting to speculate that transcription of W-specific repeats is actually the origin of chromatin compaction on the W chromosome during the life cycle. A hypothetical scheme is shown in Figure 5.…”

On the origin and functions of RNA-mediated silencing: from protists to man

“…The alternative explanation, that in these species an essential RNAi role(s) was compensated by another mechanism, would require the independent evolution of the latter multiple times. Moreover, in organisms that contain a single Dicer gene such as S. pombe and vertebrates, Dicer-null mutants are RNAi-defective but viable at the cellular level (Giraldez et al 2005;Kanellopoulou et al 2005;Martienssen et al 2005;Murchison et al 2005;Volpe et al 2002). Though these mutants commonly show reactivation of transposons and/or repetitive sequences, deficient (hetero)chromatin formation, and/or abnormal chromosome segregation; and vertebrate germ cells fail to differentiate (Fukagawa et al 2004;Kanellopoulou et al 2005;Martienssen et al 2005;Murchison et al 2005).…”

“…RNAi-dependent transcriptional silencing has been demonstrated to entail histone modifications, such as H3K9 methylation, in eukaryotes belonging to at least three different supergroups (Table 2). Moreover, siRNA-triggered transcriptional repression occurs in organisms that lack cytosine DNA methylation such as S. pombe and C. elegans (Grishok et al 2005;Martienssen et al 2005;Ponger and Li 2005;Robert et al 2005;Volpe et al 2002) and in organisms with very limited DNA methylation such as T. brucei and D. melanogaster (Kavi et al 2005;Pal-Bhadra et al 2004;Ponger and Li 2005;Shi et al 2004;Ullu et al 2004). In contrast, RNA-directed DNA methylation has, thus far, only been demonstrated in plants and mammals (Kawasaki and Taira 2004;Matzke and Birchler 2005;Morris et al 2004) and the role of DNA methylation in this type of gene silencing is somewhat debatable in mammals (Ting et al 2005;Weinberg et al 2006).…”

“…Interestingly, the mechanism(s) of RNAi-mediated (hetero)chromatin formation also appears to have diverged in present day eukaryotes since, for instance, an RdRP has been implicated in this process in S. pombe Verdel et al 2004;Volpe et al 2002) but it occurs in the absence of RdRPs in D. melanogaster and vertebrates (Fukagawa et al 2004;Kanellopoulou et al 2005;Kavi et al 2005;Pal-Bhadra et al 2004;Ting et al 2005;Weinberg et al 2006). Adding to this complexity, cytosine DNA methylation and histone modifications seem to be interconnected in self-reinforcing feedback loops in higher eukaryotes (Fuks 2005), although the role (if any) of the RNAi machinery in this cycle is not clear.…”

L’hétérochromatine constitutive dans tous ses états

“…Chez la levure S. pombe, ces ARN centromériques sont nécessaires à l'éta-blissement et au maintien de la configuration épigénétique et par conséquent à l'intégrité des centromères. En particulier, le ciblage aux centromères de l'HMT Clr4, homologue de Suv39h, et donc de la méthylation de H3K9, dépend de la machinerie d'inactivation par l'ARN (ARNi) et de la formation de petits ARN interférents (siARN) issus des régions centromériques [6,7]. L'assemblage de CENP-A, si important pour la formation du kinétochore, dépend aussi de ces siARN [8].…”