oriC also contains 11 GATC/CTAG repeats that are methylated on adenine on both strands.

TLS Online TPP Program

#Id: 6878

#Unit 3. Fundamental Processes

PRC1 complex binds the methylated nucleosomes through dimeric Pc subunits (CBXs in mammals), each containing a methyl lysine–binding domain (called a chromodomain) specific for methylated H3 lysine 27. 

Binding of the dimeric Pc to neighboring nucleosomes is proposed to condense the chromatin into a structure that inhibits transcription.

TLS Online TPP Program

#Id: 6879

#Unit 3. Fundamental Processes

The PRC1 complex contains a ubiquitin ligase that monoubiquitinylates histone H2A at lysine 119 in the H2A C-terminal tail

TLS Online TPP Program

#Id: 6880

#Unit 3. Fundamental Processes

Modification of H2A inhibits elongation by inhibiting a histone chaperone that removes histone octamers from DNA as Pol II transcribes through a nucleosome, then replaces them as the polymerase passes. 

TLS Online TPP Program

#Id: 6881

#Unit 3. Fundamental Processes

This mechanism is a key feature of Polycomb repression, which is maintained through successive cell divisions for the life of an organism (~100 years for some vertebrates, 2000 years for a sugar cone pine!)

TLS Online TPP Program

#Id: 6882

#Unit 3. Fundamental Processes

TLS Online TPP Program

#Id: 6883

#Unit 3. Fundamental Processes

Inactive X chromosome also becomes associated with PRC2 complexes, which catalyze the trimethylation of histone H3 lysine 27. This methylation results in association of the PRC1 complex and transcriptional repression