Genetic characterization of psp encoding the DING protein in Pseudomonas fluorescens SBW25

Psp is closely related to the periplasmic Pi binding component of the ABC-type phosphate transporter system (Pst). psp is flanked by a gene cluster predicted to function as a type II protein secretion system (Hxc). Deletion analysis combined with chromosomally integrated 'lacZ fusions showed that both psp and pstC are induced by Pi limitation and that pstC is required for competitive growth of the bacterium in Pi limited medium. hxcR is not regulated by Pi 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 hxcR mutation (ΔhxcR). A promoter fusion between hxcR and a promoterless copy of a gene ('dapB) essential for growth in the plant environment showed that expression of hxcR is elevated during colonization of sugar beet seedlings. A similar analysis of psp showed that it is not induced in the plant environment.Psp gene is expressed under conditions of Pi 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.DING proteins, named for the highly-conserved N-terminal sequence (DINGGG-), were initially described as proteins from animal and plant tissues with molecular weights about 38–40 kDa. The first DING protein was identified from human rheumatoid arthritis (RA) synovial fluid as a lymphocyte stimulatory protein [1]. Since then human DING proteins have been identified independently from urine and kidney stones as crystal adhesion inhibitors [2]; in skin fibroblasts and cervical carcinoma cells using hirudin-agarose affinity columns [3]; and in breast cells by their high affinity for genistein, an estrogen analog [4]. In these latter two cases, extracellular DING protein is linked to cell growth promotion by an autocrine or para