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 Qy 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 Qy region. The LH1RC complex purified from Sr2+-TTP cells exhibited its Qy maximum at 888 nm (B888). Depletion of Sr2+ from the B888 species induced a blue-shift of the LH1 Qy peak and decreased the thermal stability. Upon the Sr2+-reconstitution, the LH1-RC recovered its thermal stability and Qy 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.