The Adhesion-Associated sca Operon in Streptococcus gordonii Encodes an Inducible High-Affinity ABC Transporter for Mn2+ Uptake

摘要

ScaA lipoprotein in Streptococcus gordonii is a member of the LraI family of homologous polypeptides found among streptococci, pneumococci, and enterococci. It is the product of the third gene within the scaCBA operon encoding the components of an ATP-binding cassette (ABC) transporter system. Inactivation of scaC (ATP-binding protein) or scaA (substrate-binding protein) genes resulted in both impaired growth of cells and >70% inhibition of ⁵⁴Mn²⁺ uptake in media containing <0.5 μM Mn²⁺. In wild-type and scaC mutant cells, production of ScaA was induced at low concentrations of extracellular Mn²⁺ (<0.5 μM) and by the addition of ≥20 μM Zn²⁺. Sca permease-mediated uptake of ⁵⁴Mn²⁺ was inhibited by Zn²⁺ but not by Ca²⁺, Mg²⁺, Fe²⁺, or Cu²⁺. Reduced uptake of ⁵⁴Mn²⁺ by sca mutants and by wild-type cells in the presence of Zn²⁺ was abrogated by the uncoupler carbonylcyanide m-chlorophenylhydrazone, suggesting that Mn²⁺ uptake under these conditions was proton motive force dependent. The frequency of DNA-mediated transformation was reduced >20-fold in sca mutants. The addition of 0.1 mM Mn²⁺ to the transformation medium restored only partly the transformability of mutant cells, implying an alternate role for Sca proteins in the transformation process. Cells of sca mutants were unaffected in other binding properties tested and were unaffected in sensitivity to oxidants. The results show that Sca permease is a high-affinity mechanism for the acquisition of Mn²⁺ and is essential for growth of streptococci under Mn²⁺-limiting conditions.