Abstract
Two different protochlorophyllide (Pchlide) reductases, light-dependent and light-independent reductases (LPOR and DPOR, respectively), are involved in chlorophyll biosynthesis in most oxygenic phototrophs. To reveal the functional differentiation of the two enzymes on environmental oxygen level, a pair of mutant, LPOR-less and DPOR-less mutants, of the cyanobacterium Plectonema boryanum was photoautotrophically cultivated under high light conditions bubbling with N2 gas containing various O2 levels. While DPOR-less mutant grew as well as wild type under all conditions examined, LPOR-less mutant did not grow in the conditions where O2 level was more than 3%, suggesting that DPOR is an oxygen-sensitive enzyme. Western analysis for the three DPOR subunits and LPOR indicated that the ChlL and ChlN subunits are markedly increased in LPOR-less mutant. The Pchlide content of the mutant was 4-6 times higher than that of wild type. Possible involvement of Pchlide in the induction of chlL-chlN operon is also discussed.
