Expression of the pstS gene of Streptomyces lividans is regulated by the carbon source and is partially independent of the PhoP regulator

<p>Abstract</p> <p>Background</p> <p>PstS is a phosphate-binding lipoprotein that is part of the high-affinity phosphate transport system. <it>Streptomyces lividans </it>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.</p> <p>Results</p> <p>Functionality experiments revealed that this extracellular PstS protein does not have the capacity to capture phosphate and transfer it to the cell. Regulation of the <it>pstS </it>promoter was studied with Northern blot experiments, and protein levels were detected by Western blot analysis. We observed that the <it>pstS </it>gene was expressed in cultures containing glucose or fructose, but not in complex basal medium. Northern blot analyses revealed that the <it>pst </it>operon (<it>pstSCAB</it>) was transcribed as a whole, although higher transcript levels of <it>pstS </it>relative to those of the other genes of the operon (<it>pstC, pstA </it>and <it>pstB</it>) were observed. Deletion of the -329/-144 fragment of the <it>pstS </it>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 <it>pstS </it>expression that was independent of this regulator. Thus, the PhoP-independent expression of the <it>pstS </it>gene makes this system different from all those studied previously.</p> <p>Conclusion</p> <p>1.- In <it>S. lividans</it>, 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 <it>pstS </it>promoter may act as a binding site for a repressor. 3.- There is a basal expression of the <it>pstS </it>gene that is not controlled by the main regulator: PhoP.</p