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

Abstract

The light-harvesting 1 reaction center (LH1-RC) complex from Thermochromatium tepidum exhibits a largely red-shifted LH1 Q absorption at 915 nm due to binding of Ca, resulting in an "uphill" energy transfer from LH1 to the reaction center (RC). In a recent study, we developed a heterologous expression system (strain TS2) to construct a functional hybrid LH1-RC with LH1 from Tch. tepidum and the RC from Rhodobacter sphaeroides [Nagashima, K. V. P., et al. (2017) Proc. Natl. Acad. Sci. U. S. A. 114, 10906]. Here, we present detailed characterizations of the hybrid LH1-RC from strain TS2. Effects of metal cations on the phototrophic growth of strain TS2 revealed that Ca is an indispensable element for its growth, which is also true for Tch. tepidum but not for Rba. sphaeroides. The thermal stability of the TS2 LH1-RC was strongly dependent on Ca in a manner similar to that of the native Tch. tepidum, but interactions between the heterologous LH1 and RC became relatively weaker in strain TS2. A Fourier transform infrared analysis demonstrated that the Ca-binding site of TS2 LH1 was similar but not identical to that of Tch. tepidum. Steady-state and time-resolved fluorescence measurements revealed that the uphill energy transfer rate from LH1 to the RC was related to the energy gap in an order of Rba. sphaeroides, Tch. tepidum, and strain TS2; however, the quantum yields of LH1 fluorescence did not exhibit such a correlation. On the basis of these findings, we discuss the roles of Ca, interactions between LH1 and the RC from different species, and the uphill energy transfer mechanisms.

A Dual Role for Ca2+ in Expanding the Spectral Diversity and Stability of Light-Harvesting 1 Reaction Center Photocomplexes of Purple Phototrophic Bacteria

“…2−4 Interestingly, however, recent studies using a heterologously expressed hybrid LH1-RC complex reported that the uphill energy transfer rate indeed depends on the extent of the energy gap. 6,7 In some purple bacteria, decreasing the LH1 Q y transition energy is largely involved in the uphill energy gap. For example, the purple bacteria Roseospirillum (Rss.)…”

“…The ringlike LH1 encircles the RC to form an LH1-RC complex, in which uphill energy transfer from LH1 to the RC occurs against the energy barrier . The reaction mechanisms of the uphill energy transfer have been investigated by time-resolved spectroscopic measurements. − Previous studies demonstrated that the rate of uphill energy transfer in purple bacteria is essentially independent of the energy gap between the LH1 and RC complexes. − Interestingly, however, recent studies using a heterologously expressed hybrid LH1-RC complex reported that the uphill energy transfer rate indeed depends on the extent of the energy gap. , …”

A comparative look at structural variation among RC–LH1 ‘Core’ complexes present in anoxygenic phototrophic bacteria

“…This red shift correlates with the presence of tightly bound Ca 2+ ion, one co-ordinated in the space between each αβ-dimer in the C-terminal loop region, Figs. 1 c and 4 b top (Kimura et al 2008 , 2009 ). The ends of the αβ-polypeptides in RC–LH1 (and LH2) complexes are, generally, quite flexible and may not be visible in the electron density maps.…”

“…Interestingly, other divalent cations can substitute for Ca 2+ in the Tch. tepidum RC–LH1 ‘Core’ complex (Kimura et al 2018 ). The structure of this complex has also been solved with either Sr 2+ or Ba 2+ substituting for Ca 2 + (Yu et al 2016 ).…”

Photosynthetic Growth and Energy Conversion in an Engineered Phototroph Containing Thermochromatium tepidum Light-Harvesting Complex 1 and the Rhodobacter sphaeroides Reaction Center Complex

“…Our recent study showed that this “uphill” energy transfer in the hybrid strain occurs with an efficiency of 48%, which, surprisingly, is even greater than that observed in Rba. sphaeroides wild type (40%) . However, an unanswered question is whether this high efficiency shown in vitro is also reflected in vivo as measured by phototrophic growth.…”