Abstract
BackgroundPstS is a phosphate-binding lipoprotein that is part of the high-affinity phosphate transport system. Streptomyces lividans accumulates high amounts of the PstS protein in the supernatant of liquid cultures grown in the presence of different carbon sources, such as fructose or mannose, but not in the presence of glucose or in basal complex medium.ResultsFunctionality experiments revealed that this extracellular PstS protein does not have the capacity to capture phosphate and transfer it to the cell. Regulation of the pstS promoter was studied with Northern blot experiments, and protein levels were detected by Western blot analysis. We observed that the pstS gene was expressed in cultures containing glucose or fructose, but not in complex basal medium. Northern blot analyses revealed that the pst operon (pstSCAB) was transcribed as a whole, although higher transcript levels of pstS relative to those of the other genes of the operon (pstC, pstA and pstB) were observed. Deletion of the -329/-144 fragment of the pstS promoter, including eight degenerated repeats of a sequence of 12 nucleotides, resulted in a two-fold increase in the expression of this promoter, suggesting a regulatory role for this region. Additionally, deletion of the fragment corresponding to the Pho boxes recognized by the PhoP regulator (from nucleotide -141 to -113) resulted in constitutive pstS expression that was independent of this regulator. Thus, the PhoP-independent expression of the pstS gene makes this system different from all those studied previously.Conclusion1.- In S. lividans, only the PstS protein bound to the cell has the capacity to bind phosphate and transfer it there, whereas the PstS form accumulated in the supernatant lacks this capacity. 2.- The stretch of eight degenerated repeats present in the pstS promoter may act as a binding site for a repressor. 3.- There is a basal expression of the pstS gene that is not controlled by the main regulator: PhoP.
Highlights
PstS is a phosphate-binding lipoprotein that is part of the high-affinity phosphate transport system
Our previous observations showed that the PstS protein was strongly accumulated in the supernatants of S. lividans cultures grown in the presence of certain carbon sources
The pstS promoter drives the expression of an in-frame fusion between the DNA fragment of the xysA gene encoding the signal peptide of the xylanase Xys1 from S. halstedii JM8 [13] and the region of the pstS gene that encodes the secreted form of the PstS protein
Summary
PstS is a phosphate-binding lipoprotein that is part of the high-affinity phosphate transport system. A high expression of the PstS protein occurs under stress conditions, including alkali-acid conditions, the addition of subinhibitory concentrations of penicillin, and the response of pathogenic bacteria to the eukaryotic intracellular environment [8,9,10,11]. All these observations suggest that PstS would be one of the multi-emergency proteins that help cells to adapt to growth in different habitats
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