The binding affinity of the precursor of the small subunit of ribuloseo-1,5-bisphosphate carboxylase (pSS) to isolated, intact chloroplasts and to isolated chloroplast envelopes from the green alga Chlamydomonas reinhardii was studied under conditions where no import into chloroplasts occurred, pSS bound to both chloroplasts and envelopes with equally high affinity, The dissociation constants were 5.9 ± 2.1×10−9 M and 2.9 ± 1.4× 10−9 M, respectively. The number of binding sites per chloroplast was determined to be 8.1 ± 4.1×104. 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.
