The periplasmic component of the DctPQM TRAP-transporter is part of the DctS/DctR sensory pathway in Rhodobacter sphaeroides.

Rhodobacter sphaeroides can use C4-dicarboxylic acids to grow heterotrophically or photoheterotropically, and it was previously demonstrated in Rhodobacter capsulatus that the DctPQM transporter system is essential to support growth using these organic acids under heterotrophic but not under photoheterotrophic conditions. In this work we show that in R. sphaeroides this transporter system is essential for photoheterotrophic and heterotrophic growth, when C4-dicarboxylic acids are used as a carbon source. We also found that over-expression of dctPQM is detrimental for photoheterotrophic growth in the presence of succinic acid in the culture medium. In agreement with this, we observed a reduction of the dctPQM promoter activity in cells growing under these conditions, indicating that the amount of DctPQM needs to be reduced under photoheterotrophic growth. It has been reported that the two-component system DctS and DctR activates the expression of dctPQM. Our results demonstrate that in the absence of DctR, dctPQM is still expressed albeit at a low level. In this work, we have found that the periplasmic component of the transporter system, DctP, has a role in both transport and in signalling the DctS/DctR two-component system.