The Phn system of Mycobacterium smegmatis: a second high-affinity ABC-transporter for phosphate.

Uptake of inorganic phosphate, an essential but often limiting nutrient, in bacteria is usually accomplished by the high-affinity ABC-transport system Pst. Pathogenic species of mycobacteria contain several copies of the genes encoding the Pst system (pstSCAB), and two of the encoded proteins, PstS1 and PstS2, have been shown to be virulence factors in Mycobacterium tuberculosis. The fast-growing Mycobacterium smegmatis contains only a single copy of the pst operon. This study reports the biochemical and molecular characterization of a second high-affinity phosphate transport system, designated Phn. The Phn system is encoded by a three-gene operon that constitutes the components of a putative ABC-type phosphonate/phosphate transport system. Expression studies using phnD- and pstS-lacZ transcriptional fusions showed that both operons were induced when the culture entered phosphate limitation, indicating a role for both systems in phosphate uptake at low extracellular concentrations. Deletion mutants in either phnD or pstS failed to grow in minimal medium with a 10 mM phosphate concentration, while the isogenic wild-type strain mc(2)155 grew at micromolar phosphate concentrations. Analysis of the kinetics of phosphate transport in the wild-type and mutant strains led to the proposal that the Phn and Pst systems are both high-affinity phosphate transporters with similar affinities for phosphate (i.e. apparent K(m) values between 40 and 90 muM P(i)). The Phn system of M. smegmatis appears to be unique in that, unlike previously identified Phn systems, it does not recognize phosphonates or phosphite as substrates.