An Arabidopsis thaliana cDNA encoding PS II‐X, a 4.1 kDa component of photosystem II: a bipartite presequence mediates SecA/ΔpH‐independent targeting into thylakoids

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Abstract
Higher plant photosystem II preparations contain a 4.1 kDa polypeptide (subunit X) associated with the oxygen‐evolving core complex. We describe the isolation of a cDNA encoding PS II‐X from Arabidopsis thaliana, in which the C‐terminal region is highly homologous to partially sequenced PS II‐X from wheat and spinach. The mature protein of 42 residues is preceded by a 74‐residue, bipartite presequence similar to those involved in the targeting of nuclear‐encoded thylakoid lumen proteins, although hydrophobicity analysis indicates the presence of a single transmembrane span in the mature protein. Moreover, import of pre‐PS II‐X into the thylakoid membrane of isolated chloroplasts is unaffected by inhibitors of either the Sec‐ or ΔpH‐dependent thylakoidal protein translocases, suggesting a spontaneous insertion mechanism. PS II‐X appears to be encoded as a mature protein by the plastid genome in the chlorophyll a + c‐containing alga, Odontella sinensis. We thus propose that the thylakoid transfer signal of Arabidopsis pre‐PS II‐X represents a recent acquisition, in phylogenetic terms, compared with signals of Sec‐dependent lumenal proteins.