Coordinated evolution of oxygenic photosynthesis and chlorophyll biosynthesis

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Protochlorophyllide (Pchlide) oxidoreductase catalyzes the penultimate step of chlorophyll a biosynthesis. Most oxygenic phototrophs including cyanobacteria have two structurally unrelated Pchlide reductases, a light-dependent Pchlide reductase(LPOR) and a nitrogenase-like light-independent (dark-operative) Pchlide reductase (DPOR). In this study, we investigate the following two aspects ; 1)biochemical properties of DPOR from Rhodobacter capsulatus, and 2)functional differentiation of DPOR and LPOR in response to environmental changes of light and oxygen levels in cyanobacteria.1)We demonstrated some basic properties of DPOR from Rhodobacter capsulatus. The Km value for Pchlide was 10.6 μM. Ferredoxin functioned as an electron donor to DPOR. Elution profiles in gel filtration chromatography indicated that L-protein and NB-protein are a homodimer [(BchL)_2] and a heterotetramer [(BchN)_2(BchB)_2], respectively. 2)A maximal oxygen level for photoautotrophic growth of a cyanobacterial LPOR-lacking mutant, in which DPOR is the sole Pchlide reductase, was determined. We propose the oxygen level (3% v/v) "Chlorophyll Pasteur point". A soluble fraction was prepared from the LPOR-lacking mutant cells grown under high light bubbling with nitrogen (containing 20 CO_2) in an anaerobic chamber. The DPOR activity was successfully detected in the soluble fraction in an ATP-dependent manner for the first time. Upon exposure to the air, the DPOR activity was drastically decreased with a half-life of approximately 15 min, which was comparable with that of DPOR from R.capsulatus. This result suggests that oxygenic phototrophic organisms such as cyanobacteria have evolved some mechanisms to protect DPOR from oxygen.