Abstract
Ferredoxin-NADP
+
oxidoreductase (FNR) catalyzing the terminal step of the linear photosynthetic electron transport was purified from the cyanobacterium
Spirulina platensis
and the red alga
Cyanidium caldarium.
FNR of
Spirulina
consisted of three domains (CpcD-like domain, FAD-binding domain, and NADP
+
-binding domain) with a molecular mass of 46 kDa and was localized in either phycobilisomes or thylakoid membranes. The membrane-bound FNR with 46 kDa was solublized by NaCl and the solublized FNR had an apparent molecular mass of 90 kDa. FNR of
Cyanidium
consisted of two domains (FAD-binding domain and NADP
+
-binding domain) with a molecular mass of 33 kDa. In
Cyanidium
, FNR was found on thylakoid membranes, but there was no FNR on phycobilisomes. The membrane-bound FNR of
Cyanidium
was not solublized by NaCl, suggesting the enzyme is tightly bound in the membrane. Although both cyanobacteria and red algae are photoautotrophic organisms bearing phycobilisomes as light harvesting complexes, FNR localization and membrane-binding characteristics were different. These results suggest that FNR binding to phycobilisomes is not characteristic for all phycobilisome retaining oxygenic photosynthetic organisms, and that the rhodoplast of red algae had possibly originated from a cyanobacterium ancestor, whose FNR lacked the CpcD-like domain.