A temperature-dependent conformational change in photosystem-II thylakoid membrane

Abstract

A photosystemsubchloroplast fragment (TSF-2a) fractionated from spinach chloroplasts by Triton X-l 00 treatment is highly enriched in cytochrome bss9 (cyt b& (%1/40 total chlorophyll (chl) molecules) [ 11. The cytochrome in this subchloroplast fragment is present in the low-potential form, with E7sn -60 mV [2]. The oxidized form of cyt bss9 in the isolated subchloroplast fragments readily undergoes photoreduction, which is followed by a slower reversal to the oxidized form in the dark [2]. Cyt bss9 in isolated chloroplasts has rather anomalous properties and its physiological function is yet unclear ([3-61 also cf. [7]). However, it is generally considered that cyt bss9 is closely associated with photosystem II. Detergent-fractionated subchloroplast fragments, in which the state of cyt bss9 has probably been modified as a result of exposure to the detergent, do provide useful vehicles for investigating its physico-chemical and biochemical properties, particularly the relationship between them, which may be relevant to the functional role of the cytochrome in photosynthesis. This note reports the finding of a temperaturedependent change in the oxidation kinetics of cyt bss9 which indicates the effect of a possible conformational change in the microenvironment in the thylakoid membrane.