A Spectroscopic Variant of the Light-Harvesting 1 Core Complex from the Thermophilic Purple Sulfur Bacterium Thermochromatium tepidum

Abstract

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, Ca(2+). Here, we report a spectroscopic variant of LH1RC complexes from Tch. tepidum cells in which Ca(2+) was biosynthetically replaced with Sr(2+). The photosynthetic growth of wild-type cells cannot be maintained without Ca(2+) and is heavily inhibited when the Ca(2+) is replaced with other metal cations. Interestingly, only Sr(2+) supported photosynthetic growth instead of Ca(2+) 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 Sr(2+) to ∼870 nm (B870). Reconstitution of Sr(2+) and Ca(2+) 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.

Ca2+-binding reduces conformational flexibility of RC–LH1 core complex from thermophile Thermochromatium tepidum

“…However, this correlation seems not to hold for manganese- (Fig. 2) or strontium-substituted complexes (Kimura et al 2011). Our results thus confirm that Ca 2?…”

“…to *887 nm. The LH1 Q y absorption has been previously shown to be red-shifted by metal cations up to 918 nm (Kimura et al 2008), and very recently also blue shifting has been reported (Kimura et al 2011). Photosynth Res (2012 The thermal stability of RC-LH1 complexes is monitored by the CD signal of metal depleted and substituted complexes in b-OG during heat denaturation (Fig.…”

Biochemical and Spectroscopic Characterizations of a Hybrid Light-Harvesting Reaction Center Core Complex

“…tepidum. 18 Therefore, in contrast to the case for Tch. tepidum, Sr 2+ is not a functional replacement for Ca 2+ in strain TS2 and cannot support normal phototrophic growth of this chimeric strain.…”

“…tepidum. 18 An additional band at 863 nm was also present that likely originates from RC-only complexes that were not assembled into LH1-RC complexes. These results suggest that Sr 2+ can be incorporated into the LH1-RC complexes of strain TS2 in place of Ca 2+ , although the population of functional Sr 2+ -substituted complexes was much lower than that observed in biosynthetically Sr 2+ -substituted LH1-RC complexes from Tch.…”

“…tepidum LH1-RC complexes was conducted as described previously. 18 LH1-only complexes from TS2 and DP2 were prepared as follows. The chromatophores of TS2 and DP2 were treated with 0.23−0.25% LDAO in 20 mM Tris-HCl buffer at pH 7.5 and 8.5, respectively, for 60 min, followed by ultracentrifugation at 4 °C and 196000g for 60 min.…”