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Fig. 7 | Epigenetics & Chromatin

Fig. 7

From: 27nt-RNAs guide histone variant deposition via ‘RNA-induced DNA replication interference’ and thus transmit parental genome partitioning in Stylonychia

Fig. 7

The electron micrograph (kindly donated by Ada and Don Olins) below shows a section from a polytene chromosome from developing macronuclei in Stylonychia. Tight heterochromatic bands are interconnected by uncondensed chromatin fibres. The right-handed cartoon is an interpretation of that micrograph within a model for ‘RNA-induced DNA replication interference’ through 27nt-RNAs. 27nt-RNAs protect DNA from being lost during macronuclear development. Accordingly, PIWI1/27nt-RNA complexes block polytenization of covered MDSs, thus limiting the replication-dependent de novo deposition of nucleosomes. Histone variant H3.4 is probably permissive for H3K27me3. H3K27me3 nucleosomes become enriched during a restricted timeframe. Eventually, this tight spatiotemporal coordination leads to the association of H3(.4)K27me3 preferentially with bulk DNA—besides H3.5 that is continuously expressed. The heterochromatin formation via H3K27me3 is limited to those regions not protected by PIWI1/27nt-RNA complexes

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Epigenetics & Chromatin

ISSN: 1756-8935