No abstract

No abstract

An intrinsic 22 kDa polypeptide is found associated with the oxygen-evolvin~ photosystem 11 (PSII) core complex in all green plants and cyanobaeteria so far examined, although it does not appear to be required l'or oxygen evolution. Amino acid ~equenee itfforrnation obtained from the purified 22 kDa protein was used to construct a probe that was employed to isolate a full-length eDNA clone encoding the 274-residue precursor of the 22 kDa protein. Hydropathy plot analysis predicts the existence of four membrane-spanning helices in the mature protein, The two halves of the approxlmntely 200-resld uc mature protein show high sequence similarity to each other, suggesting that the psbS gene arose from an internal gene duplication. The 22 kDa protein has some sequence similarity to chlorophyll a/b.binding proteins.

Photosystem II reaction centers comprised of the Dl, D2 and cytochrome b-559 polypeptides were isolated from welldefined oxygen-evolving reaction center preparations using either a combination of LiClO, and dodecyhnaltoside, or Triton X-100 alone. Yields of chlorophyll and cytochrome b-559 for both preparative methods were compared, and pigment contents were compared based on 2.0 b-559 per reaction center, a standard derived from photosystem II components found in the starting material. Results obtained with dodecylmaltoside suggest that the reaction center of photosystem II binds 10-12 Chl a along with 2 Cyt b-559 molecules. Reaction centers prepared with Triton X-100 bind about 8 Chl a per 2 Cyt b-559 molecules, a finding which indicates that Triton X-100 extracts pigments from the reaction center polypeptides. Our results contradict the widely held notion that the pigment binding properties of the photosystem II reaction center are similar to those of the bacterial reaction center.

The primary electron donor (P700) in Photosystem I (PSI) has been shown to be a dimeric chlorophyll a species. Electron magnetic resonance studies of the cation radical have clearly established that the unpaired electron is delocalized asymmetrically over this dimer; however, the extent to which this asymmetry exists remains ambiguous. Comprehensive electron nuclear double resonance (ENDOR) and electron spin−echo envelope modulation (ESEEM) experiments combined with isotopic substitution and numerical simulations have been used to determine the electronic structure of P700 + . This approach utilizes the strengths of each spectroscopy to elucidate the electron nuclear hyperfine and nuclear quadrupole coupling constants for the nitrogen nuclei in P700 + . These assignments are then confirmed by performing numerical simulations of the ESEEM data. Further confirmation of these values is obtained by performing the spin−echo experiments at multiple microwave frequencies. The same set of hyperfine and quadrupole coupling constants is used to simulate all of the ESEEM data for P700 + containing either natural abundance 14N or isotopically enriched with 15N. These simulations indicate that the unpaired spin is localized over only one of the chlorophylls that make up the special pair. The ramifications of this monomeric spin density on the function and thermodynamics of electron transfer in PSI are discussed.