The nuclear‐encoded polypeptide Cfo‐II from spinach is a real, ninth subunit of chloroplast ATP synthase

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Abstract
Proton‐translocating F‐ATP synthases from chloroplasts contain a nuclear‐coded subunit, CFo‐II, that lacks an equivalent in the corresponding E. coli complex. Three recombinant phages that code for the entire precursor of this subunit have been isolated from λ gt11 cDNA expression libraries made from polyadenylated spinach RNA using a two‐step strategy. The reading frame of 222 amino acid residues includes 147 residues for the mature protein (M r 16.5 kDa) and a transit sequence of 75 residues (M r 8.0 kDa). Secondary structure predictions indicate a bitopic protein, anchored by a single N‐terminal transmembrane segment and a C‐terminal hydrophilic region that probably reaches into CF1. CFo‐II precursor made in vitro can be imported into isolated, intact chloroplasts and assembled into ATP synthase. This protein is a real subunit of the plastid enzyme and a distinctive characteristic of ATP synthases involved in photosynthetic processes. Unique features are (i) that the gene for CFo‐II (atpG) appears to be a duplication of atpF encoding CFo‐I, the homologues of the genes for subunits b′ and b in photosynthetic bacteria, (ii) that it represents the first instance that one copy of the various duplicated loci found in plastid chromosomes has been phylogenetically translocated to the nucleus, and (iii) that it operates with a bipartite (import/thylakoid‐targeting) transit peptide but without an intermediate cleavage site for the stroma protease, suggestive of a way of membrane integration different from that of its plastome‐encoded counterpart CFo‐I. With these data, the first complete sequence for a chloroplast ATP synthase of a higher plant (spinach) is available.