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NifU-like proteins are a highly conserved protein that serves as the scaffold for assembly of Fe-S clusters. Chloroplastic NifU-like proteins have tandem NifU like domains, named domain I and domain II. Although the amino acid sequences of these domains are very similar to each other, the predicted functional region for the Fe-S cluster assembly, the CXXC motif, exists only in domain I. The structure of the domain II of chloroplastic NifU-like protein OsNifU1A has an alpha-beta sandwich structure containing two alpha helices located on one side of the beta-sheet. The electrostatic surface potential of OsNifU1A domain II is predominantly positively charged. Chloroplastic NifU-like proteins are targeted to ferredoxin for transferring the Fe-S cluster. The ferredoxin presents an overall negatively charged surface, which may evoke an electrostatic association with OsNifU1A domain II.

Effects of ultraviolet irradiation of chloroplasts on several photochemical reactions mediated by Photosystem II were studied at room temperature. The Hill activity and fluorescence of variable yield were decreased by ultraviolet irradiation in parallel. The activity of 2,6-dichlorophenolindophenol (DCIP) photoreduction in irradiated chloroplasts was only slightly recovered by addition of diphenylcarbazide, an electron donor for Photosystem II. No restoration of the original fluorescence yield was observed on addition of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) or preincubating the irradiated chloroplasts with dithionite. Photobleaching of C-550 was much more resistant than the Hill activity or fluorescence of variable yield toward ultraviolet irradiation. Chloroplasts, of which the Hill activity and variable fluorescence had mostly been eliminated by ultraviolet irradiation, still showed C-550 photobleaching, which was in magnitude more than 50% of that in the original unirradiated chloroplasts. C-550 was shown not to respond to membrane potential. The photobleaching of C-550 is sensitized by Photosystem II and proceeds with unaltered quantum efficiency in ultraviolet-irradiated chloroplasts. These observations are interpreted to indicate that C-550 is not identical with Q, the hypothetical quencher of fluorescence, and that C-550 is a better indicator for the primary photoact in Photosystem II than Q.

Light -induced absorbance changes at 560 nm in dark-adapted intact chloroplasts of the green alga, Bryopsis maxima were studied in the time range of 200 ms. The initial rise of the 560 nm signals consists of two major components which are both electrochromic absorbance changes of the carotenoids, siponein and/or siphonaxanthin, but different in mechanisms of the field formation. The first component (component S) is related to electron transport since it was sensitive to 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB) and showed at light-intensity dependence similar to that of electron transport in chloroplasts. In the presence of DCMU, component S could be restored on addition of proton-transporting electron donors such as reduced 2.6-dichlorophenol indophenol and phenazine methosulfate, but not on addition of N,N,N',N'-tetramethyl-p-phenylenediamine which does not carry protons with electrons (Trebst, A. (1974) Annu. Rev. Plant Physiol. 25, 423--458). We propose that component S is due to the electric field set up by the proton translocation across the thylakoid membrane. The second component (component R) was resistant to DCMU and DBMIB. The light-intensity dependency of component R was similar to that of cytochrome f photooxidation which showed saturation at a relatively low light intensity. The magnitude of component R was markedly reduced by phenylmercuric acetate, suggesting the participation of ferredoxin and ferredoxin-NADP oxidoreductase in the mechanism of the field formation responsible for this component. In the presence of DCMU and phenylmercuric acetate, time courses of the 560 nm changes paralleled those of cytochrome f changes. These results indicate that component R is due to the electric field formed between oxidized cytochrome f and other intersystem electron carriers located in the inner part of the thylakoid membrane and reduced electron acceptors of Photosystem I situated on the membrane surface. The complex natures of the 560 nm changes, as well as the contributions of Photosystems I and II to the absorbance changes, are explained in terms of the two electrogenic mechanisms.