Plasticity in cell defence: access to and reactivity of critical protein residues and DNA response elements.

Cellular and whole organ defence against pathogenic or chemical challenge is manifest as an adaptive response. Where appropriate, this may lead to induction of a cellular defence programme, thereby enhancing cell survival. When the challenge is overwhelming, the defence is breached and a switch is made to yield cell death, either by apoptosis or necrosis. Thus, a cell will defend itself where possible, but in extremis, it may recognise the futility of its resistance and allow itself to die. Transcription factor activation and access to the DNA regulatory elements that control a particular pattern of expression of defence genes is a major issue that may ultimately decide the fate of a cell in a changed environment. It is possible to visualise the access to the nucleus and to the genome, of paradigm gene loci or transcription factors, using a number of molecular techniques such as chromatin immunoprecipitation, in vivo footprinting and live/whole cell imaging. These methods are informative as to the array of transcription factors that may regulate a given gene, as well as the transitory nature of the transcriptional activation. The initial triggering of active transcription factor complexes typically occurs within the cytoplasm of the cell. Protein-protein interactions and signal transduction pathways, elucidated using a classical molecular genetics approach, have long been recognised as pivotal to the initial control of the levels and activity of transcription factors. We can now visualise modifications in critical residues of transcription factors and regulators during cellular response to chemical stress. These modifications may yield enhanced or repressed activity of transcription factors, they may be non-covalent or covalent, and they may occur in response to a variety of classes of chemicals. Such promiscuous signalling can provide plasticity in the cellular response to a wide array of chemical agents.