Typically, intracellular signalling molecules are activated by modification, e.g. phosphorylation or methylation, and interact in complicated biochemical reaction networks.
Activation of RpoN-dependent transcription requires the presence of EBPs activated by modification such as phosphorylation, and therefore, an increase in the amount of RpoN alone would not directly lead to increasing expression of every RpoN-regulated gene.
To mediate these functions, the p53 protein must be activated by posttranslational modifications, which modulate p53 activity and interactions.
PGC-1α is activated by posttranslational modifications, including phosphorylation and deacetylation.
It has also been suggested that ligands can be activated by certain modifications during the recycling process [ 3– 6].
When leaf senescence is induced, the WRKY53 locus is activated by histone modifications H3K4me2 and H3K4me3 (Ay et al., 2009; Brusslan et al., 2012), whereas DNA methylation remains low and unchanged (Zentgraf et al., 2010).
Furthermore, Parp-1 is a structural component of the transcriptionally repressed state of chromatin, and transcription is reported to be activated by auto-modification activity in an NAD-dependent manner [ 12].
They may be activated by covalent modification in response to the accumulation of unfolded proteins in the mitochondrial matrix or this may lead to transcriptional activation of genes encoding MURE1 and 2, a process that would require the intervention of yet another transcription factor.
As described earlier TRPA1 is activated by covalent modification of cysteine residues.
p53 is activated by post-translational modifications, and the protein is phosphorylated at multiple amino acids [17].
