Inhibition of plastocyanin to P-700(+) electron transfer in Chlamydomonas reinhardtii by hyperosmotic stress

Oxygen electrode and fluorescence studies demonstrate that linear electron transport in the freshwater alga Chlamydomonas reinhardtii can be completel y abolished by abrupt hyperosmotic shock. We show that the most likely prim ary site of inhibition of electron transfer by hyperosmotic shock is a bloc kage of electron transfer between plastocyanin (PC) or cytochrome c(6) and P-700. The effects on this reaction were reversible upon dilution of the os molytes and the stability of plastocyanin or photosystem (PS) I was unaffec ted. Electron micrographs of osmotically shocked cells showed a significant decrease in the thylakoid lumen volume. Comparison of estimated lumenal wi dth with the x-ray structures of plastocyanin and PS I suggest that lumenal space contracts during HOS so as to hinder the movement of docking to PS I of plastocyanin or cytochrome c(6).