Variations on a theme: Polycomb group proteins in plants

Abstract

Polycomb group (PcG) proteins evolved early in evolution, probably in the common ancestor of animals and plants. In some unicellular organisms, such as Chlamydomonas and Tetrahymena, PcG proteins silence genes in heterochromatin, suggesting an ancestral function in genome defence. In angiosperms, the PcG system controls many developmental transitions. A PcG function in the vernalization response evolved especially in Brassicaceaea. Thus, the role of PcG proteins has changed during evolution to match novel needs. Recent studies identified many proteins associated with plant PcG protein complexes. Possible functions of these interactions are discussed here. We highlight recent findings about recruitment of PcG proteins in plants in comparison with animal system. Through the new data, a picture emerges in which PcG protein complexes do not function in sequential linear pathways but as dynamically interacting networks allowing stabilizing feedback loops. We discuss how the interplay between different PcG protein complexes can enable establishment, maintenance, and epigenetic inheritance of H3K27me3.

Gene silencing in microalgae: Mechanisms and biological roles

“…The methylation of lysine residues in histones, with the exception of H3K79 methylation, is carried out by enzymes that contain an evolutionary conserved SET domain, named after three Drosophila genes (Su(var)3-9, Enhancer of zeste, and Trithorax) (Casas-Mollano et al, 2007;Bannister and Kouzarides, 2011;Huang et al, 2011;Derkacheva and Hennig, 2014). The SET domain constitutes the catalytic site of these lysine methyltransferases (KMTs), but flanking sequences, more distant protein domains, and possibly some cofactors are also important for enzyme activity and specificity (Huang et al, 2011;Krishnan et al, 2011;Derkacheva and Hennig, 2014).…”

“…The SET domain constitutes the catalytic site of these lysine methyltransferases (KMTs), but flanking sequences, more distant protein domains, and possibly some cofactors are also important for enzyme activity and specificity (Huang et al, 2011;Krishnan et al, 2011;Derkacheva and Hennig, 2014). To begin characterizing the occurrence and the role(s) of H3K9 and/or H3K27 methyltransferases in microalgae, we surveyed 14 complete or nearcomplete algal genomes in the Archaeplastida super group for the presence of SETdomain polypeptides (Table 1).…”

“…Members of the algal KMT6 class, like animal and plant KMT6 proteins, are likely responsible for H3K27 methylation and putative components of the evolutionarily conserved Polycomb Repressive Complex 2 (Shaver et al, 2010;Bannister and Kouzarides, 2011;Huang et al, 2011;Derkacheva and Hennig, 2014). Within the examined microalgae, KMT6 homologs appear to be widely distributed (Table 1 and Fig.…”

“…By analogy to their function in higher eukaryotes (Casas-Mollano et al, 2007;Krauss, 2008;Shaver et al, 2010;Bannister and Kouzarides, 2011;Huang et al, 2011;Derkacheva and Hennig, 2014), these post-translational histone modifications are likely involved in transcriptional repression in microalgae. Yet, experimental evidence is almost entirely lacking.…”

“…These functionally and structurally different chromatin states are marked by distinct covalent modifications on the DNA and on specific amino acid residues of the nucleosomal histones (Casas-Mollano et al, 2007;Bannister and Kouzarides, 2011;Saze and Kakutani, 2011). For instance, di-or trimethylation of histone H3 lysine 9 (H3K9) or of histone H3 lysine 27 (H3K27) is often associated with silenced chromatin (Krauss, 2008;Shaver et al, 2010;Bannister and Kouzarides, 2011;Saze and Kakutani, 2011;Derkacheva and Hennig, 2014). DNA cytosine methylation also plays a role in repression and in some organisms there appears to be a complex interplay between histone tail modifications and DNA methylation in establishing a silent chromatin structure (Law and Jacobsen, 2010;Saze and Kakutani, 2011;Du et al, 2012;Zhong et al, 2014).…”