Rhodopsin with its activation by retinal photoisomerization is likely to correspond to an induced-fit scenario for initial events, whereas both scenarios are conceivable for later activation phases as well as for GPCRs activated by diffusible ligands.
Interestingly, increased CCL2 and PCa metastasis was observed in TRAMP mice with AR ablation in either prostate epithelial cells or macrophages (Niu et al, 2008), supporting that CCL2 expression triggered by AR silencing in either cell type could be an initiating signal for later activation of the CCL2/STAT3/EMT signalling pathways.
However, in a few cell types, mRNAs with short poly(A) tails are stored for later activation by re-polyadenylation in the cytosol.
While these methylation events on H3 have clear contrasting effects on gene expression, it is believed that genes marked with both H3K4me3 and H3K27me3 remain in a "poised" state ready for later activation 46.
Slight heterochronic changes in hybrid development (for example, later activation of transcriptional machinery) may manifest themselves as widespread under-expression or even over-expression of larval genes.
Hence, these divergent observations have led to hypothesize that in stem cells PcG proteins target both genes poised for later activation and genes poised for later repression, acting as a common platform that clearly requires additional gene- and lineage-specific signals when differentiation unfolds [6], [9].
In fact, the first model (repressed but poised for later activation) implies that most of the cells harbour coexisting repressive and activating marks and that both PcG and trxG have to be maintained at the given locus through cell divisions to keep the repressed-but-poised status until further changes in the balance of their activity.
Their argument last night had either been forgotten or stored for later activation.
The bivalent modification was proposed to keep the key developmental genes in poised states for later activation.
In undifferentiated ES cells this unusual combination of marks is thought to keep genes repressed or expressed at very low levels but poised for later activation.
10.7554/eLife.00861.010 Figure 3. Widespread Pol II pausing of MBT genes and poising for later activation.
