lo2 Role of PcG mediated histone modifications

[?] What is the catalytic function of PRC1?

PRC1 (Polycomb Repressive Complex 1) catalyses mono-ubiquitination of Histone H2A at lysine 119 (H2AK119ub1).

[?] What is the catalytic function of PRC2?

PRC2 (Polycomb Repressive Complex 2) catalyses mono/di/tri-methylation of H3 at lysine 27 (H3K27me1/2/3).

[?] What is the function of the PR-DUB complex?

PR-DUB (Polycomb Repressive Deubiquitinase complex) deubiquitinates Histone H2A at lysine 119, removing the ubiquitin mark added by PRC1.

[?] How were PRC1 and PRC2 complexes identified?

Mass spectrometry studies allowed the identification of complexes responsible for H2AK119ub1 and H3K27me3, eventually purifying the PRC1 and PRC2 complexes respectively.

[?] Is H2AK119ub1 essential for transcriptional repression?

Yes, studies that specifically remove PRC1 catalytic activity showed that H2AK119ub1 is instructive in transcriptional repression. Derepression of PcG target genes happens as quickly as 2 hours after PRC1 removal.

[?] Is H3K27 methylation essential for Polycomb repression?

Yes, H3K27 methylation is essential for Polycomb repression.

[?] What happens when H2AK119ub1 is lost in organisms?

Studies in Drosophila and mouse reported that while H2AK119ub1 is not absolutely required for PcG repression of HOX genes, the organisms were inviable, highlighting the developmental importance of PcG. Residual PRC1 activity remained in these animals.

[?] What are some histone modifications associated with active transcription?

H3K27ac is found at active promoters and enhancers, H3K4me3 is found at active promoters, and H3K4me1 is associated with enhancers.

[?] What histone modification is associated with centromeres?

H3K9me3 is found at centromeres and promotes HP1 gamma bind