During phosphate-limited growth the marine phycoerythrin-containing picoplanktonic cyanobacterium Synechococcus sp. WH7803 synthesizes novel polypeptides, including two abundant species of 100 kDa and 32 kDa. The 32 kDa polypeptide was localized to the cell wall, although in a related strain, Synechococcus sp. WH8103, it could be detected in both the cell wall fraction and the periplasm. The gene (designated pstS) encoding this polypeptide was cloned and shown to be present in a single copy. The deduced amino acid sequence indicated a polypeptide consisting of 326 amino acids with a calculated M(r) of 33,763. Comparison of this sequence with that obtained by microsequencing the N-terminus of the 32 kDa polypeptide showed that the mature protein was synthesized as a precursor, the first 24 amino acid residues being cleaved between two alanine residues at positions 24 and 25. The amino acid sequence of the mature polypeptide showed 35% identity and 52% similarity to the periplasmic phosphate-binding protein (PstS) from Escherichia coli, including three regions of much stronger homology which, by comparison with E. coli PstS, are directly involved in phosphate binding. Northern blot analysis revealed a pstS transcript of 1.2 kb in RNA extracted from cells grown in Pi-replete conditions and one of 1.4 kb in considerably increased abundance under Pi-depleted conditions. Homologues of the pstS gene were detected in other marine phycoerythrin-containing Synechococcus strains, but not in freshwater or halotolerant species.