Secondary transporters of citrate in complex with metal ions belong to the bacterial CitMHS family, about which little is known. The transport of metal-citrate complexes in Streptomyces coelicolor has been investigated. The best cofactor for citrate uptake in Streptomyces coelicolor is Fe3+, but uptake was also noted for Ca2+, Pb2+, Ba2+, and Mn2+. Uptake was not observed with the Mg2+, Ni 2+, or Co2+ cofactor. The transportation of iron-and calcium-citrate makes these systems unique among the CitMHS family members reported to date. No complementary uptake akin to that observed for the CitH (Ca2+, Ba2+, Sr2+) and CitM (Mg2+, Ni2+, Mn2+, Co2+, Zn2+) systems of Bacillus subtilis was noted. Competitive experiments using EGTA confirmed that metal-citrate complex formation promoted citrate uptake. Uptake of free citrate was not observed. The open reading frame postulated as being responsible for the metal-citrate transport observed in Streptomyces coelicolor was cloned and overexpressed in Escherichia coli strains with the primary Fe 3+-citrate transport system (fecABCDE) removed. Functional expression was successful, with uptake of Ca2+-citrate, Fe3+- citrate, and Pb2+-citrate observed. No free-citrate transport was observed in IPTG (isopropyl-β-D-thiogalactopyranoside)-induced or -uninduced E. coli. Metabolism of the Fe3+-citrate and Ca 2+-citrate complexes, but not the Pb2+-citrate complex, was observed. Rationalization is based on the difference in metal-complex coordination upon binding of the metal by citrate.
ASJC Scopus subject areas
- Molecular Biology