The low-affinity phosphate transporter PitA is dispensable for in vitro growth of Mycobacterium smegmatis

Mycobacteria have been shown to contain an apparent redundancy of high-affinity phosphate uptake systems, with two to four copies of such systems encoded in all mycobacterial genomes sequenced to date. In addition, all mycobacteria also contain at least one gene encoding the low-affinity phosphate transporter, Pit. No information is available on a Pit system from a Gram-positive microorganism, and the importance of this system in a background of multiple other phosphate transporters is unclear.

The aim of this study was to determine the physiological role of the PitA phosphate transporter in Mycobacterium smegmatis. Expression of pitA was found to be constitutive under a variety of growth conditions. An unmarked deletion mutant in pitA of M. smegmatis was created. The deletion did not affect in vitro growth or phosphate uptake of M. smegmatis. Expression of the high-affinity transporters, PstSCAB and PhnDCE, was increased in the pitA deletion strain.

PitA is the only low-affinity phosphate transport system annotated in the genome of M. smegmatis. The lack of phenotype of the pitA deletion strain shows that this system is dispensable for in vitro growth of this organism. However, increased expression of the remaining phosphate transporters in the mutant indicates a compensatory mechanism and implies that PitA is indeed used for the uptake of phosphate in M. smegmatis.