Formation of prolamellar-body-like ultrastructures in etiolated cyanobacterial cells overexpressing light-dependent protochlorophyllide oxidoreductase in Leptolyngbya boryana

Abstract

Protochlorophyllide (Pchlide) reduction is the penultimate step of chlorophyll (Chl) biosynthesis, and is catalyzed by two evolutionarily unrelated enzymes: dark-operative Pchlide oxidoreductase (DPOR) and light-dependent Pchlide oxidoreductase (LPOR). Because LPOR is the sole Pchlide reductase in angiosperms, dark-grown seedlings of angiosperms become etiolated. LPOR exists as a ternary complex of Pchlide-NADPH-LPOR to form paracrystalline prolamellar bodies (PLBs) in etioplasts. Because LPOR is distributed ubiquitously across oxygenic phototrophs including cyanobacteria, it would be important to determine whether cyanobacterial LPOR has the ability to form PLBs. We isolated a DPOR-less transformant ΔchlL/LPORox, carrying a plasmid to overexpress cyanobacterial LPOR in the cyanobacterium Leptolyngbya boryana. The transformant did not produce Chl in the dark and became etiolated with an accumulation of Pchlide and LPOR. Novel PLB-like ultrastructures were observed in etiolated cells, which disappeared during the early stage of the light-dependent greening process. However, the rate of Chl production in the greening process of ΔchlL/LPORox was almost the same as that observed in the control cells, which carried an empty vector. An in vitro LPOR assay of extracts of dark-grown ΔchlL/LPORox cells suggested that the PLB-like structures are deficient in NADPH. Low-temperature fluorescence emission spectra of membrane fractions of the etiolated cells indicated the absence of the photoactive form of Pchlide, which was consistent with the inefficiency of the greening process. Cyanobacterial LPOR exhibited an intrinsic ability to form PLB-like ultrastructures in the presence of the co-accumulation of Pchlide; however, the PLB-like structure differed from the authentic PLB regarding NADPH deficiency.