Newly synthesized PsbO is reversibly attached to the thylakoid membrane and released from the membrane under light stress

The 33 kDa protein (PsbO) is an important component of the photosystem II r eaction center and located at the lumenal side of the thylakoid membrane. i n vivo labelling assays of tobacco seedlings and subsequent immunoprecipita tion of PsbO revealed that newly synthesized PsbO is present in the lumen a s soluble and membrane-associated forms. The latter form appears to be asso ciated with photosystem II. Since membrane association also occurs in darkn ess and in the presence of chloramphenicol, no newly-formed photosystem II particles are required. Comparable features with regard to the membrane ass ociation and membrane release of PsbO were obtained with an in vitro recons titution system. In contrast to the D1 protein, the overall amount of PsbO does not change significantly under light stress; however, the polypeptide appears to be released from the membranes and accumulates in the thylakoid lumen. Re-association with the membrane does not occur under light-stress c onditions. The conclusion is that membrane association of PsbO is sensitive to high light intensitites.