Binary flash-induced oscillations of [ 14C]DCMU binding to the photosystem II acceptor complex

Abstract

Binding of radioactively-labelled DCMU to spinach chloroplast membranes is determined following pre-illumination by single turnover saturating light flashes. When binding is assayed rapidly following inhibitor addition (< 40 s) distinct binary oscillations of [ 14C]DCMU binding can be observed. By dark-adapted samples or those, following an even number of flashes, more inhibitor is bound than after one or an odd number of flashes. During longer incubation times (2 min), [ 14C]DCMU binding becomes independent of flash preillumination. Comparison of the [ 14C]DCMU binding kinetics following one flash and the kinetics of the DCMU-induced increase of chlorophyll fluorescence rise reveals a substantial amount of binding which is not accompanied by a corresponding fluorescence rise. These data are discussed within the framework of the ‘inhibitor-plastoquinone competition model’ [FEBS Lett. (1981) 126, 277–281] and with reference to DCMU binding data derived from fluorescence measurements [Biochim. Biophys. Acta (1982) 682, 245–253].