Varying competence for protein import into chloroplasts during the cell cycle in Chlamydomonas

By studying the import of radioactively labelled small subunit of ribulose- 1,5-bisphosphate carboxylase (pSS) into chloroplasts of the green alga C, r einhardtii cw-15 protein delivery to chloroplasts was found to vary during the cell cycle. Chloroplasts were isolated from highly synchronous cultures at different time points during the cell cycle. When PSS was imported into 'young' chloroplasts isolated early in the light period about three times less pSS was processed to small subunit SS than in 'mature' chloroplasts fr om the middle of the light period. In 'young' chloroplasts also, less PSS w as bound to the envelope surface. During the second half of the light perio d the import competence of isolated chloroplasts decreased again when based on chlorophyll content or cell volume, but did not change significantly wh en related to chloroplast number. Measurements of pSS binding to the surfac e of chloroplasts of different age indicated that the adaptation of protein import competence during the cell cycle is due to a variation of the numbe r of binding sites per chloroplast surface area, rather than to modulation of the binding constant.