The type II secretion system (Xcp) ofPseudomonas putidais active and involved in the secretion of phosphatases

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The genome of the Gram-negative bacteriumPseudomonas putidaharbours a complete set ofxcpgenes for a type II protein secretion system (T2SS). This study shows that expression of these genes is induced under inorganic phosphate (Pi) limitation and that the system enables the utilization of various organic phosphate sources. A phosphatase of the PhoX family, previously designated UxpB, was identified, which was produced under low Pi conditions and transported across the cell envelope in an Xcp-dependent manner demonstrating that thexcpgenes encode an active T2SS. The signal sequence of UxpB contains a twin-arginine translocation (Tat) motif as well as a lipobox, and both processing by leader peptidase II and Tat dependency were experimentally confirmed. Two differenttatgene clusters were detected in theP. putidagenome, of which one, namedtat-1, is located adjacent to theuxpBandxcpgenes. Both Tat systems appeared to be capable of transporting the UxpB protein. However, expression of thetat-1genes was strongly induced by low Pi levels, indicating a function of this system in survival during Pi starvation.

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