Chlorophyll b is involved in long-wavelength spectral properties of light-harvesting complexes LHC I and LHC II

Chlorophyll (Chl) molecules attached to plant light-harvesting complexes (LHC) differ in their spectral behavior. While most Chl a and Chl b molecules give rise to absorption bands between 645 nm and 670 nm, some special Chls absorb at wavelengths longer than 700 nm. Among the Chl a/b-antennae of higher plants these are found exclusively in LHC I. In order to assign this special spectral property to one chlorophyll species we reconstituted LHC of both photosystem I (Lhca4) and photosystem II (Lhcb1) with carotenoids and only Chl a or Chl b and analyzed the effect on pigment binding, absorption and fluorescence properties. In both LHCs the Chl-binding sites of the omitted Chl species were occupied by the other species resulting in a constant total number of Chls in these complexes. 77-K spectroscopic measurements demonstrated that omission of Chl b in refolded Lhca4 resulted in a loss of long-wavelength absorption and 730-nm fluorescence emission. In Lhcb1 with only Chl b long-wavelength emission was preserved. These results clearly demonstrate the involvement of Chl b in establishing long-wavelength properties.

Ultrafast excitation dynamics of low energy pigments in reconstituted peripheral light-harvesting complexes of photosystem I

Ultrafast dynamics of a reconstituted Lhca4 subunit from the peripheral LHCI-730 antenna of photosystem I of higher plants were probed by femtosecond absorption spectroscopy at 77 K. Intramonomeric energy transfer from chlorophyll (Chl) b to Chl a and energy equilibration between Chl a molecules observed on the subpicosecond time scale are largely similar to subpicosecond energy equilibration processes within LHCII monomers. However, a 5 ps equilibration process in Lhca4 involves unique low energy Chls in LHCI absorbing at 705 nm. These pigments localize the excitation both in the Lhca4 subunit and in LHCI-730 heterodimers. An additional 30-50 ps equilibration process involving red pigments of Lhca4 in the heterodimer, observed by transient absorption and picosecond fluorescence spectroscopy, was ascribed to intersubunit energy transfer.

Protein domains required for formation of stable monomeric Lhca1- and Lhca4-complexes

The peripheral light-harvesting complex of Photosystem I consists of two subpopulations, LHCI-680 and LHCI-730. The latter is composed of the two apoproteins Lhca1 and Lhca4. Recently, reconstitution of monomeric LHCI using bacterially overexpressed Lhca1 or Lhca4 was achieved. In order to obtain insight into the structure requirements for formation of monomeric light-harvesting complexes, we produced a series of N- and C-terminal deletion mutants and used the overexpressed proteins for reconstitution experiments. We found the entire extrinsic N-terminal region dispensable for monomer formation in Lhca1 and Lhca4. Also at the C-terminus, both subunits revealed similarity since all amino acids up to the end of the fourth helix could be removed without abolishing monomer formation. In connection with former corresponding results for Lhcb1, the dispensability of these regions appears to be a general feature in LHC-formation. In LHCI, however, a stabilising effect can be ascribed to these regions since the yield of complexes was decreased. In the majority of the mutant LHCI versions no effect on pigment binding was detected. However, in the LHC with the most extensively N-terminally truncated mutant of Lhca4 a dramatic shift in the 77 K fluorescence emission to shorter wavelengths was observed. This suggests that chlorophylls involved in long wavelength fluorescence emission are located in the chlorophyll array located towards the stromal face of the thylakoid membrane assuming a pigment arrangement corresponding to that in LHCII and CP29.

In vitro reconstitution of the photosystem I light-harvesting complex LHCI-730: heterodimerization is required for antenna pigment organization

Here we describe the in vitro reconstitution of photosystem I light-harvesting complexes with pigments and proteins (Lhca1 and Lhca4) obtained by overexpression of tomato Lhca genes in Escherichia coli. Using Lhca1 and Lhca4 individually for reconstitution results in monomeric pigment-proteins, whereas a combination thereof yields a dimeric complex. Interactions of the apoproteins is highly specific, as reconstitution of either of the two constituent proteins in combination with a light-harvesting protein of photosystem II does not result in dimerization. The reconstituted Lhca1/4, but not complexes obtained with either Lhca1 or Lhca4 alone, closely resembles the native LHCI-730 dimer from tomato leaves with regard to spectroscopic properties, pigment composition, and stoichiometry. Monomeric complexes of Lhca1 or Lhca4 possess lower pigment/protein ratios, indicating that interactions of the two subunits not only facilitates pigment reorganization but also recruitment of additional pigments. In addition to higher averages of chlorophyll a/b ratios in monomeric complexes than in LHCI-730, comparative fluorescence and CD spectra demonstrate that heterodimerization involves preferential ligation of more chlorophyll b.

Requirement for the H phosphoprotein in photosystem II of Chlamydomonas reinhardtii

To dissect the expression of the psbB gene cluster of the Chlamydomonas reinhardtii chloroplast genome and to assess the role of the photosystem II H-phosphoprotein (PSII-H) in the biogenesis and/or stabilization of PSII, an aadA gene cassette conferring spectinomycin resistance was employed for mutagenesis. Disruption of the gene cluster has no effect on the abundance of transcripts of the upstream psbB/T locus. Likewise, interruption of psbB/T and psbH with a strong transcriptional terminator from the rbcL gene does not influence transcript accumulation. Thus, psbB/T and psbH may be independently transcribed, and the latter gene seems to have its own promoter in C. reinhardtii. In the absence of PSII-H, translation and thylakoid insertion of chloroplast PSII core proteins is unaffected, but PSII proteins do not accumulate. Because the deletion mutant also exhibits PSII deficiency when dark-grown, the effect is unrelated to photoinhibition. Turnover of proteins B and C of PSII and the polypeptides PSII protein A and PSII protein D is faster than in wild-type cells but is much slower than that observed in other PSII-deficient mutants of C. reinhardtii, suggesting a peripheral location of PSII-H in PSII. The role of PSII-H on PSII assembly was examined by sucrose gradient fractionation of pulse-labeled thylakoids; the accumulation of high-molecular-weight forms of PSII is severely impaired in the psbH deletion mutant. Thus, a primary role of PSII-H may be to facilitate PSII assembly/stability through dimerization. PSII-H phosphorylation, which possibly occurs at two sites, may also be germane to its role in regulating PSII structure, stabilization, or activity.