Chloroplast import and sequential maturation of pea carbonic anhydrase: the roles of various parts of the transit peptide

Chloroplast pea carbonic anhydrase is synthesised in the cytosol with an unusually long bipartite N-terminal extension of the mature sequence previously proposed to serve as a transit peptide. Studies of import into pea chloroplasts show that the N-terminal 69 amino acids of the previously proposed transit peptide is sufficient for translocation and localisation to the stroma, while the acidic C-terminal part does not seem to have any function in these processes. Processing of the in vitro imported precursors is shown to be at a new cleavage site located in the middle of the actual transit peptide. The results indicate that maturation occurs in more than one step. The time-course does not seem to be dependent on the age of the chloroplast but on the age of the translocated precursor.