Strontium Ions are Functionally Replaceable with Calcium Ions in the Light-Harvesting 1 Reaction Center Core Complex from Thermophilic Purple Sulfur Bacterium Thermochromatium Tepidum

Abstract

Thermochromatium tepidum (TTP) is a themophilic purple sulfur photosynthetic bacterium of which light-harvesting 1 reaction center (LH1RC) complexes exhibit an unusual LH1 Q_y absorption at 915 nm and thermal resistance relative to those of mesophilic counterparts. Recently, we demonstrated that these unique properties were regulated by an inorganic cofactor, Ca²⁺. Wild-type TTP cells grew anaerobically at 50 °C in a culture medium containing a small amount of CaCl₂. When Ca²⁺ was removed from the medium or replaced with other metal cations, the photosynthetic growth was largely suppressed, however, only Sr²⁺ was biofunctionally replaceable with Ca²⁺. The resulting Sr²⁺-substituted TTP (Sr²⁺-TTP) cells showed different spectral properties compared with the native ones in the LH1 Q_y region. The LH1RC complex purified from Sr²⁺-TTP cells exhibited its Q_y maximum at 888 nm (B888). Depletion of Sr²⁺ from the B888 species induced a blue-shift of the LH1 Q_y peak and decreased the thermal stability. Upon the Sr²⁺-reconstitution, the LH1-RC recovered its thermal stability and Q_y peak position depending on the Sr²⁺-concentration with an approximately 10⁴ M⁻¹ of binding constant. This is the first evidence for a functional LH1-variant which was prepared by the biosynthetic metal substitution.