| Popis: |
The ImpX transporters of the Drug/Metabolite Transporter (DMT) superfamily were first proposed to transport riboflavin (vitamin B2) based on findings of a cis-regulatory RNA element responding to flavin mononucleotide (an FMN riboswitch). BdeHovibrio exovorous JSS has a homolog belonging to this superfamily. It has 10 TMSs and shows 30% identity to the previously characterized ImpX transporter from Fusobacterium nucleatum. However, the ImpX homolog is not regulated by an FMN-riboswitch. In order to test the putative function of the ImpX homolog from B. exovorous (BexImpX), we cloned and heterologously expressed its gene. We used functional complementation, growth inhibition experiments, direct uptake experiments and inhibition studies, suggesting a high degree of specificity for riboflavin uptake. The EC50 for growth with riboflavin was estimated to be in the range 0.5–1 μM, estimated from the half-maximal riboflavin concentration supporting the growth of a riboflavin auxotrophic E. coli strain, but the K(half) for riboflavin uptake was 20 μM. Transport experiments suggested that the energy source is the proton motive force, but that NaCl stimulates uptake. Thus, members of the ImpX family members are capable of riboflavin uptake, not only in riboflavin prototrophic species such as F. nucleatum, but also in the B2 auxotrophic species, B. exovorous. |
