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Summary: Hypothesis
A redox switch hypothesis for the origin of two
light reactions in photosynthesis
John F. Allen*
School of Biological Sciences, Queen Mary, University of London, Mile End Road, London E1 4NS, UK
Received 28 December 2004; accepted 4 January 2005
Available online 21 January 2005
Edited by Richard Cogdell
Abstract Photosynthesis provides energy in the EarthÕs bio-
sphere and oxygen in its atmosphere. For oxygen to be produced,
two different light reactions must operate simultaneously and in
series. Known anaerobic, photosynthetic bacteria contain one or
other of these photosystems, but never both. Here, I propose that
the two photosystems diverged, in structure and function, from a
common ancestor, within a single, continuous, anaerobic lineage.
In such cells, living examples of which are predicted, the two
photosystems are isoenzymes encoded by orthologous genes un-
der co-ordinated, redox regulatory control. A redox switch re-
sponds to defined environmental conditions and selects which
set of genes is expressed. In these cells, the two photosystems
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