SummaryThe oxidation of methionine residues in proteins to methionine sulfoxides occurs frequently and protein repair by reduction of the methionine sulfoxides is mediated by an enzyme, peptide methionine sulfoxide reductase (PMSR, EC 1.8.4.6), universally present in the genomes of all so far sequenced organisms. Recently, ®ve PMSR-like genes were identi®ed in Arabidopsis thaliana, including one plastidic isoform, chloroplast localised plastidial peptide methionine sulfoxide reductase (pPMSR) that was chloroplast-localized and highly expressed in actively photosynthesizing tissue (Sadanandom A et al., 2000). However, no endogenous substrate to the pPMSR was identi®ed. Here we report that a set of highly conserved methionine residues in Hsp21, a chloroplast-localized small heat shock protein, can become sulfoxidized and thereafter reduced back to methionines by this pPMSR. The pPMSR activity was evaluated using recombinantly expressed pPMSR and Hsp21 from Arabidopsis thaliana and a direct detection of methionine sulfoxides in Hsp21 by mass spectrometry. The pPMSR-catalyzed reduction of Hsp21 methionine sulfoxides occurred on a minute time-scale, was ultimately DTT-dependent and led to recovery of Hsp21 conformation and chaperone-like activity, both of which are lost upon methionine sulfoxidation (Ha È rndahl et al., 2001). These data indicate that one important function of pPMSR may be to prevent inactivation of Hsp21 by methionine sulfoxidation, since small heat shock proteins are crucial for cellular resistance to oxidative stress.

A model for the topological organization of the chloroplast thylakoid membrane is presented. A series of illustrations is provided, which outline a suggested 3-dimensional structure in cross-section and in full shape, which accounts both for the folding of one continuous membrane into multiple grana stacks as seen in cross-sectional electron micrographs and for the rapid reversible unfolding (destacking) of the grana stacks into lamellar sheets.

No abstract

No abstract

Wild-type and an atrazine-resistant biotype of Brassica napus, in which a glycine is substituted for the serine-264 of the D1 protein, were grown over a wide range of constant irradiances in a growth cabinet. In the absence of serine-264, the function of photosystem I1 (PSII) was changed as reflected by changes in chlorophyll fluorescence parameters and in photosynthetic oxygen-evolving activity. lhe photochemical quenching coefficient was lower, showing that a larger proportion of the primary quinone acceptor is reduced at all irradiances. At low actinic irradiances, the nonphotochemical quenching coefficient was higher, showing a greater tendency for heat emission. Decreased rates of light-limited photosynthesis (quantum yield) and lower oxygen yields per single-turnover flash were also observed. lhese changes were observed even when the plants had been grown under low irradi-ances, indicating that the changes in PSll function are direct and not consequences of photoinhibition. In spite of the lowered PSll efficiency under light-limiting conditions, the light-saturated pho-tosynthesis rate of the atrazine-resistant mutant was similar to that of the wild type. An enhanced susceptibility to photoinhibition was observed for the atrazine-resistant biotype compared to the wild type when plants were grown under high and intermediate, but not low, irradiance. We conclude that the replacement of serine by glycine in the D1 protein has a direct effect on PSll function, which in turn causes increased photoinhibitory damage and increased rates of turnover of the D1 protein. 60th the intrinsic lowering of light-limited photosynthetic efficiency and the increased sensitivity to photoinhibition probably contribute to reduced crop yields in the field, to different extents, depending on growth conditions. The PSII reaction center of green plants performs one of the key steps in the process of photosynthetic energy conversion. The heart of the PSII reaction center comprises a heterodimer of D1 and D2 proteins, with known primary sequences, to which a11 of the redox components are bound, as has been recently deduced from the close analogy with the crystallized reaction center of photosynthetic bacteria (* Corresponding author; fax 46-46-104534. 1991). Upon absorption of a photon by P680, charge separation takes place, which can result either in photochemistry if stabilized by QA reduction or in heat emission via a back reaction. After transfer of the electron from QA-to QB and then to the plastoquinone pool, the reaction center is ready to utilize a second photon. The D1 protein in the PSII reaction center heterodimer has a light-dependent and rapid rate of tumover (Mattoo et al., 1984), which is related to the phenomenon of photoinhibition (Prasil et al., 1992). Continuous repair of PSII is believed to be necessary because of the involvement of highly oxidizing radicals and toxic oxygen species (Barber and Andersson, 1991; Prasil et al., 1992). The study of effects of single amino acid substitutions has become an important app...