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 sequenc e 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 an d localisation to the stroma, while the acidic C-terminal part does no t seem to have any function in these processes. Processing of the in v itro imported precursors is shown to be at a new cleavage site located in the middle of the actual transit peptide. The results indicate tha t maturation occurs in more than one step. The time-course does not se em to be dependent on the age of the chloroplast but on the age of the translocated precursor.