Editing G0 phase (section)

===Rb and G<sub>0</sub> exit===
Studies suggest that Rb repression of the [[E2F]] family of transcription factors regulates the G<sub>0</sub> to G<sub>1</sub> transition just as it does the G<sub>1</sub> to S transition. Activating E2F complexes are associated with the recruitment of [[histone acetyltransferases]], which activate gene expression necessary for G<sub>1</sub> entry, while [[E2F4]] complexes recruit histone deacetylases, which repress gene expression. Phosphorylation of Rb by Cdk complexes allows its dissociation from E2F transcription factors and the subsequent expression of genes necessary for G<sub>0</sub> exit. Other members of the Rb [[pocket protein family]], such as p107 and p130, have also been found to be involved in G<sub>0</sub> arrest. p130 levels are elevated in G<sub>0</sub> and have been found to associate with E2F-4 complexes to repress transcription of E2F target genes. Meanwhile, p107 has been found to rescue the cell arrest phenotype after loss of Rb even though p107 is expressed at comparatively low levels in G<sub>0</sub> cells. Taken together, these findings suggest that Rb repression of E2F transcription factors promotes cell arrest while phosphorylation of Rb leads to G<sub>0</sub> exit via derepression of E2F target genes.<ref name="Sage2004"/> In addition to its regulation of E2F, Rb has also been shown to suppress [[RNA polymerase I]] and [[RNA polymerase III]], which are involved in [[rRNA]] synthesis. Thus, phosphorylation of Rb also allows activation of rRNA synthesis, which is crucial for protein synthesis upon entry into G<sub>1</sub>.<ref name="RenRollins2004"/>