THE RESPONSE OF THE PICOPLANKTONIC MARINE CYANOBACTERIUM SYNECHOCOCCUS SPECIES WH7803 TO PHOSPHATE STARVATION INVOLVES A PROTEIN HOMOLOGOUSTO THE PERIPLASMIC PHOSPHATE-BINDING PROTEIN OF ESCHERICHIA-COLI

During phosphate-limited growth the marine phycoerythrin-containing pi coplanktonic cyanobacterium Synechococcus sp. WH7803 synthesizes novel polypeptides, including two abundant species of 1 00 kDa and 32 kDa. The 32 kDa polypeptide was localized to the cell wall, although in a r elated strain, Synechococcus sp. WH8103, it could be detected in both the cell wall fraction and the periplasm. The gene (designated pstS) e ncoding this polypeptide was cloned and shown to be present in a singl e copy. The deduced amino acid sequence indicated a polypeptide consis ting of 326 amino acids with a calculated M(r) of 33763. 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 betwee n two alanine residues at positions 24 and 25. The amino acid sequence of the mature polypeptide showed 35% identity and 52% similarity to t he periplasmic phosphate-binding protein (PstS) from Escherichia coli, including three regions of much stronger homology which, by compariso n with E. coli PstS, are directly involved in phosphate binding. North ern blot analysis revealed a pstS transcript of 1.2 kb in RNA extracte d from cells grown in P(i)-replete conditions and one of 1.4 kb in con siderably increased abundance under P(i)-depleted conditions. Homologu es of the pstS gene were detected in other marine phycoerythrin-contai ning Synechococcus strains, but not in freshwater or halotolerant spec ies.