Characterization of compatible solute transporter multiplicity in Corynebacterium glutamicum

The soil bacterium Corynebacterium glutamicum is efficiently protected against hyperosmotic stress by a high redundancy of uptake systems and biosynthesis pathways for compatible solutes. We have previously identified and analyzed four osmoregulated uptake systems for betaine, ectoine, and proline. Because of overlapping substrate specificities, it is not possible to quantify their individual contribution to the stress response in wild-type cells. Using a set of strains in which only one uptake system for compatible solutes is present, we investigated the expression regulation at their transcriptional and translational level. The carrier ectP was found to be regulated at the level of transcription, but the already high maximal uptake capacity of approx. 30 nmol/(min mg cell dry mass, cdm) was not further elevated if the medium osmolality was severely increased, indicating that the amount of EctP is not changed. Thus, EctP may represent the rescue system for C. glutamicum. The betP, lcoP, and proP genes were induced upon hyperosmotic conditions, resulting in a 3-10-fold increase of their transport activity. These systems are thus used to fine-tune the uptake capacity for compatible solutes to the actual demands of the cell. ProP represents the most strongly regulated compatible solute uptake system in C. glutamicum.