Light, time and micro-organisms



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By analogy with pH, the negative logarithm of H+ concentration, the physicist Martin Kamen once introduced the pts scale:

pts = -log10t(s)

where t(s) is the lifetime of a process, measured in seconds.

Here Kamen's pts scale, originally intended to provide a framework for early events in photosynthesis, is brought up-to-date in the light of structural information about light-harvesting complexes, reaction centres, electron transport components, and coupling F-ATPases. The scale is also considerably extended, being made to encompass not only the early events of photosynthesis (starting with light absorption at pts = +15) but also the evolutionary origin of oxygenic photosynthesis and the contemporary origin of eukaryotic cells (pts = -6.8). It is proposed that coincidences of the latter type did not occur by chance. The role of semiquinone anion radicals in proton-motive Q-cycles (they are produced at about pts = +3.2 and removed at pts = +4.2) is a fundamental dilemma for living cells, and it is suggested that evolutionary "attempts" to resolve this dilemma account for the origins of eukaryotism and sex.

At the centre of the pts scale lie biological adaptations - physiological; developmental; and evolutionary. These adapations occur in response to the same environmental signals, but differ with respect to the levels of gene expression over which control is exerted when the response occurs. For physiology (post-translation modification of pre-existing proteins) and development (de novo synthesis of proteins) the question then arises: how do living cell make a "decision" about the appropriate level of response to be deployed? Certainly such decisions are made - trees do not shed their leaves every evening, at dusk. The recent discovery of biological clocks in the prokaryotic cyanobacteria, and their molecular characterisation, suggest the outlines of a molecular mechanism by which the output from an internal oscillator may arbitrate between competing pathways of signal transduction. This mechanism is supported by recent findings in Plant Cell Biology on two-component systems that may couple gene expression, at different levels, to the oxidation-reduction reactions of biological energy transduction in photosynthesis and respiration. The oscillator itself is always phase-locked by light signals, suggesting an evolutionary origin of all biological clocks from mechanisms of reponse to regularly repeating, circadian changes in the physical light environment.

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