| A proposal for control of post-translational covalent modification by phosphorylation and, in parallel, of gene expression: bifurcated signal transduction. If the putative redox sensor contains both sensor and response regulator domains, and if the kinetically favoured pathway for phosphoryl transfer (k1) is intramolecular, then phosphorylation of the sensor's own aspartate would initiate a physiological response, for example by activation of a protein kinase. If full activation of the kinase then still fails to restore redox poise, then phosphoryl transfer (k2) may occur to the aspartate of the response regulator that regulates gene expression. An assumption of the proposed mechanism is that intramolecular phosphoryl group transfer is kinetically favoured over intermolecular transfer: k1 > k2 . In another graphic, an additional possibility is suggested, which is that biological clocks provide an input that arbitrates beween physiological and developmental responses. The input could achieve this effect by acting on the sensor and response regulators in such a way as to alter the ratio k1:k2. |