A phosphate-starvation-inducible outer-membrane protein of Ag1, expressed at phosphate concentrations below 0.08—0.13 mM, was purified and characterized. The purification method involved separation of outer-membrane proteins by SDS-PAGE and extraction of the protein from nitrocellulose or PVDF membranes after electrotransfer of proteins to the membranes. The N-terminal amino acid sequence of the purified protein, called Psi1, did not show homology to any known proteins, and in contrast to the phosphate-specific porin OprP of its mobility in SDS-PAGE was not affected by solubilization temperature. An antiserum against Psi1 recognized a protein of 55000 in four other strains among 24 tested strains representing rRNA homology group I, showing antigenic heterogeneity within this group. A method for immunofluorescence microscopy involving cell permeabilization was adapted to visualize cell-specific expression of Psi1 in exposed to limiting amounts of phosphate. This approach should be useful for further exploration of Psi1 as a marker to study the availability of phosphate to in natural environments.


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