The ~~~d~n~ aftMy of the pr~~~~ of the small subunit of tihul~-1,5-bispho~bate ea&oxylase (pSS) to isolated, intact chlompiasts and to isolated chloroplast envelopes from the green alga C!~lunq~/a~~onas r&&z& was studied under conditions where na, import into ~~o~o~!~~ occurred. pSS bound to both chloroplasts and envelopes with equally high affinity, The dissociation constants were 5.9 & Z.IXIU~ M and 2,9 + I .4x IO-' M. res~tiv~Iy~ The tlumber of binding sires per th~o~op~t was determined to be 8.1 4 4.1~100. Binding of pSS to isolated envelopes or intact chloroplasts was specific with respect to the type of the membrane and the presence of the transit sequence.

The precursor of the small subunit of ribulose-l,5-bisphosphate carboxylase (pSS) and a modified pSS containing a C-terminal hexahistidyl tail (pSS(His)6) were imported into isolated Chlamydomonas chloroplasts with comparable efficiency. In the presence of Ni 2+ ions the import of pSS(His)6 was inhibited and the precursor bound to the envelope remained protease sensitive, while import of pSS was not affected. Addition of an excess of L-histidine suppressed the inhibition demonstrating that the hexahistidyl-Ni 2+ complex was responsible for import inhibition. Inhibition could be observed between about 0.5 and l0 mM Ni ~+, depending on the total protein content in the assay. Import incompetent Ni2+-precursor complexes can be used to study early events in chloroplast protein import.