The small subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is a major chloroplast stromal protein that is cytosolically synthesized as a precursor with an N-terminal extension, known as the transit sequence or transit peptide (Tp). The Tp is essential for the post-translational uptake of the precursor by the chloroplast. The Tp is thought to influence the conformation of the precursor protein and to facilitate polypeptide translocation across the chloroplast envelope barrier via a Tp-selective translocon. To address these issues we have devised a novel strategy to generate substrate amounts of a chloroplast targeting sequence as a fusion with the chromogenic globular domain of cytochrome b5 (Cyt). The chimaeric protein is an ideal probe for investigating the conformation of a preprotein and events surrounding protein import into isolated chloroplasts. The Cyt of liver endoplasmic reticulum was fused at its N-terminus with the Tp of the small subunit of Rubisco of Pisum sativum (pea). To enhance its production by clearance from the cytoplasm of Escherichia coli, the chimaera was engineered by further N-terminal linkage of a prokaryotic secretory signal. Expression of this tripartite fusion resulted in mg quantities of the signal sequence–processed Tp–Cyt protein, which was eventually targeted to the membranes. The chromogenic nature of the chimaera and its localization to the bacterial membrane facilitated the biochemical isolation of the precursor in a soluble and functional form. The purified preprotein displayed spectral and enzymic properties that were indistinguishable from the native parental Cyt, implying an absence of observable influence of the Tp on the conformation of the haemoprotein. The chimaeric precursor was imported into the stroma of the isolated chloroplasts in a dose-dependent manner. Import was also strongly dependent upon exogenously supplied ATP. The stromally imported chimaeric precursor protein was processed to a size characteristic of Cyt.

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