A spectroscopic variant of the light-harvesting 1 core complex from the thermophilic purple sulfur bacterium thermochromatium tepidum

Thermochromatium tepidum is a purple sulfur photosynthetic bacterium, and its light-harvesting 1 reaction center (LH1RC) complexes exhibit an unusual LH1 Q_y absorption at 915 nm (B915) and possess enhanced thermal stability. These unique properties are closely related to an inorganic cofactor, Ca²⁺. Here, we report a spectroscopic variant of LH1RC complexes from Tch. tepidum cells in which Ca²⁺ was biosynthetically replaced with Sr²⁺. The photosynthetic growth of wild-type cells cannot be maintained without Ca²⁺ and is heavily inhibited when the Ca ²⁺ is replaced with other metal cations. Interestingly, only Sr ²⁺ supported photosynthetic growth instead of Ca²⁺ with slightly reduced rates. The resulting Sr-tepidum cells exhibited characteristic absorption spectra in the LH1 Q_y region with different LH1RC:LH2 ratios depending on the growth conditions. LH1RC complexes purified from the Sr-tepidum cells exhibited a Q_y maximum at 888 nm (B888) that was blue-shifted after removal of Sr²⁺ to ∼870 nm (B870). Reconstitution of Sr²⁺ and Ca²⁺ into B870 resulted in red shifts of the Q_y peak to 888 and 908 nm, respectively. The thermal stability of B888 was slightly lower than that of B915 as revealed by differential scanning calorimetry analysis. Effects of other divalent metal cations on the Q_y peak position and thermal stability of B888 were similar but not identical to those of B915. This study provides the first evidence of a purple bacterium in which LH1RC complexes alter spectroscopic and thermodynamic properties in vivo by utilizing exogenous metal cations and improve the ability to adapt to the environmental changes.