Signal Transduction and 'Second Messengers'
In animals as well as in plants, the cellular machinery for protein synthesis responds to external stimuli. These external stimuli may include a whole range of diverse factors including the following : (i) the presence of hormones, growth factors and other signalling molecules (e.g. in animals); (ii) the presence of pheromones secreted by cells of two different mating types (i.e. in yeast); (iii) presence of light (different wave lengths), gravity, minerals, water, gases and specific soil structure (e.g. in plants) and (iv) cell-cell contact. In some of these cases, cell surface receptor molecules are present and transmit the signal to the interior of the cell (for water-soluble signals like proteins, peptides and neurotransmitters). In other cases, substances like hormones interact with intracellular receptors, which transmit the signal further to the target site (for water insoluble lipophilic signals like steroids, thyroid hormones, retinoic acid, etc.). In all these cases (including prokaryotes), the external stimulus needs to be transduced and integrated with internal signals, which follow a cascade of events called- 'signal transduction pathway' -leading to the response, which is a specific gene expression in many cases (Fig. 37.22).

Signal transduction pathways precede most of the mechanisms for regulation of gene expression discussed earlier. In prokaryotes, often the precursor molecule (e.g. lactose) or the end product (e.g. an amino acid) of a biosynthetic pathway takes part in regulation of gene activity. But in 'signal transduction pathways' in eukaryotes a cascade of molecules, leading to the activation of one or more specific transcription factors, is involved in regulation of gene activity.