Abstract
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). Here we show in-vitro activity of cyanobacterial DPOR and its oxygen sensitivity for the first time. An LPOR-lacking mutant of the cyanobacterium Plectonema boryanum was cultivated under high light bubbling with nitrogen (containing 2% CO2), and the soluble fraction was prepared in an anaerobic chamber. The Pchlide reduction activity was successfully detected in the soluble fraction in an ATP-dependent manner. Upon exposure to the air, the DPOR activity in the soluble fraction was drastically decreased with a half-life of approximately 15 min. Oxygen sensitivity of DPORs from cyanobacteria and photosynthetic bacteria will be discussed in view of the evolution of oxygenic photosynthesis.
