Natural selection will favor those individuals that allocate their resources in a way that achieves maximum fitness.

TLS Online TPP Program

#Id: 6876

#Unit 3. Fundamental Processes

Most Polycomb proteins are subunits of one of two classes of multiprotein Polycomb repressive complexes: PRC1 and PRC2.

TLS Online TPP Program

#Id: 6877

#Unit 3. Fundamental Processes

The PRC2 complexes contain histone deacetylases that inhibit transcription. They also contain a subunit [E(z) in Drosophila, EZH2 in mammals] with a SET domain, which is the catalytic domain of several histone methyl transferases.

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!)