| Summary: | <p>Abstract</p> <p>Background</p> <p>DING proteins constitute a conserved and broadly distributed set of proteins found in bacteria, fungi, plants and animals (including humans). Characterization of DING proteins from animal and plant tissues indicated ligand-binding ability suggesting a role for DING proteins in cell signaling and biomineralization. Surprisingly, the genes encoding DING proteins in eukaryotes have not been identified in the eukaryotic genome or EST databases. Recent discovery of a DING homologue (named Psp here) in the genome of <it>Pseudomonas fluorescens </it>SBW25 provided a unique opportunity to investigate the physiological roles of DING proteins. <it>P. fluorescens </it>SBW25 is a model bacterium that can efficiently colonize plant surfaces and enhance plant health. In this report we genetically characterize Psp with a focus on conditions under which <it>psp </it>is expressed and the protein exported.</p> <p>Results</p> <p>Psp is closely related to the periplasmic P<sub>i </sub>binding component of the ABC-type phosphate transporter system (Pst). <it>psp </it>is flanked by a gene cluster predicted to function as a type II protein secretion system (Hxc). Deletion analysis combined with chromosomally integrated '<it>lacZ </it>fusions showed that both <it>psp </it>and <it>pstC </it>are induced by P<sub>i </sub>limitation and that <it>pstC </it>is required for competitive growth of the bacterium in P<sub>i </sub>limited medium. <it>hxcR </it>is not regulated by P<sub>i </sub>limitation. Psp was detected (using anti-DING serum) in the supernatant of wild-type culture but was greatly reduced in the supernatant of an isogenic strain carrying an <it>hxcR </it>mutation (Δ<it>hxcR</it>). A promoter fusion between <it>hxcR </it>and a promoterless copy of a gene ('<it>dapB</it>) essential for growth in the plant environment showed that expression of <it>hxcR </it>is elevated during colonization of sugar beet seedlings. A similar analysis of <it>psp </it>showed that it is not induced in the plant environment.</p> <p>Conclusion</p> <p>Psp gene is expressed under conditions of P<sub>i </sub>limitation. It is an exoprotein secreted mainly via the Hxc type II secretion system, whose expression is elevated on plant surfaces. We propose that Psp is involved in extracellular scavenging of phosphates, which are subsequently taken up by the cell-bound Pst transport system.</p>
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