RibM from Streptomyces davawensis is a riboflavin/roseoflavin transporter and may be useful for the optimization of riboflavin production strains

Abstract Background The bacterium Bacillus subtilis , which is not a natural riboflavin overproducer, has been converted into an excellent production strain by classical mutagenesis and metabolic engineering. To our knowledge, the enhancement of riboflavin excretion from the cytoplasm of overproducing cells has not yet been considered as a target for (further) strain improvement. Here we evaluate the flavin transporter RibM from Streptomyces davawensis with respect to improvement of a riboflavin production strain. Results The gene ribM from S. davawensis , coding for a putative facilitator of riboflavin uptake, was codon optimized ( ribM opt ) for expression in B. subtilis . The gene ribM opt was functionally introduced into B. subtilis using the isopropyl-β-thiogalactopyranoside (IPTG)-inducible expression plasmid pHT01: Northern-blot analysis of total RNA from IPTG treated recombinant B. subtilis cells revealed a ribM opt specific transcript. Western blot analysis showed that the his 6 -tagged heterologous gene product RibM was present in the cytoplasmic membrane. Expression of ribM in Escherichia coli increased [ 14 C]riboflavin uptake, which was not affected by the protonophore carbonyl cyanide m -chlorophenylhydrazone (CCCP). Expression of ribM opt supported growth of a B. subtilis Δ ribB ::Erm r Δ ribU ::Kan r double mutant deficient in riboflavin synthesis (Δ ribB ) and also deficient with respect to riboflavin uptake (Δ ribU ). Expression of ribM opt increased roseoflavin (a toxic riboflavin analog produced by S. davawensis ) sensitivity of a B. subtilis Δ ribU ::Kan r strain. Riboflavin synthesis by a model riboflavin B. subtilis production strain overproducing RibM was increased significantly depending on the amount of the inducer IPTG. Conclusions The energy independent flavin facilitator RibM could in principle catalyze riboflavin export and thus may be useful to increase the riboflavin yield in a riboflavin production process using a recombinant RibM overproducing B. subtilis strain (or any other microorganism).