Functional Domains of the Ferredoxin Transit Sequence Involved in Chloroplast Import

In order to analyze the information content of a chloroplast transit sequence, we have constructed and analyzed by in vitro assays seven substitution and 20 deletion mutants of the ferredoxin transit sequence. The N-terminal part and the C-terminal part are important for targeting, and in addition the C-terminal region is required for processing. A third region is important for translocation but not for the initial interaction with the envelope. A fourth region is less essential for in vitro import. Purified precursors were tested for their ability to compete for the in vitro import of radiolabeled wild-type precursor, which confirmed the important role in chloroplast recognition of both the N- and the C-terminal domain of the transit sequence. Monolayer experiments showed that the N terminus was mainly involved in the insertion into mono-galactolipid-containing lipid surfaces whereas the C terminus mediates the recognition of negatively charged lipids. A sequence comparison to other transit sequences suggests that the domain structure of the ferredoxin transit sequence can be extended to these sequences and thus reveals a general structural design of transit sequences.