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

Unravelling the Roles of Integral Polypeptides in Excitation Energy Transfer of Photosynthetic RC-LH1 Supercomplexes

Elucidating the photosynthetic processes that occur within the reaction center-light-harvesting 1 (RC-LH1) supercomplexes from purple bacteria is crucial for uncovering the assembly and functional mechanisms of natural photosynthetic systems and underpinning the development of artificial photosynthesis. Here, we examined excitation energy transfer of various RC-LH1 supercomplexes of Rhodobacter sphaeroides using transient absorption spectroscopy, coupled with lifetime density analysis, and studied the roles of the integral transmembrane polypeptides, PufX and PufY, in energy transfer within the RC-LH1 core complex. Our results show that the absence of PufX increases both the LH1 → RC excitation energy transfer lifetime and distribution due to the role of PufX in defining the interaction and orientation of the RC within the LH1 ring. While the absence of PufY leads to the conformational shift of several LH1 subunits toward the RC, it does not result in a marked change in the excitation energy transfer lifetime.

Rhodobacter capsulatus forms a compact crescent-shaped LH1-RC photocomplex

Rhodobacter (Rba.) capsulatus has been a favored model for studies of all aspects of bacterial photosynthesis. This purple phototroph contains PufX, a polypeptide crucial for dimerization of the light-harvesting 1-reaction center (LH1-RC) complex, but lacks protein-U, a U-shaped polypeptide in the LH1-RC of its close relative Rba. sphaeroides. Here we present a cryo-EM structure of the Rba. capsulatus LH1-RC purified by DEAE chromatography. The crescent-shaped LH1-RC exhibits a compact structure containing only 10 LH1 αβ-subunits. Four αβ-subunits corresponding to those adjacent to protein-U in Rba. sphaeroides were absent. PufX in Rba. capsulatus exhibits a unique conformation in its N-terminus that self-associates with amino acids in its own transmembrane domain and interacts with nearby polypeptides, preventing it from interacting with proteins in other complexes and forming dimeric structures. These features are discussed in relation to the minimal requirements for the formation of LH1-RC monomers and dimers, the spectroscopic behavior of both the LH1 and RC, and the bioenergetics of energy transfer from LH1 to the RC.

A Ca2+-binding motif underlies the unusual properties of certain photosynthetic bacterial core light-harvesting complexes

The mildly thermophilic purple phototrophic bacterium Allochromatium tepidum provides a unique model for investigating various intermediate phenotypes observed between those of thermophilic and mesophilic counterparts. The core light-harvesting (LH1) complex from A. tepidum exhibits an absorption maximum at 890 nm and mildly enhanced thermostability, both of which are Ca²⁺-dependent. However, it is unknown what structural determinants might contribute to these properties. Here, we present a cryo-EM structure of the reaction center–associated LH1 complex at 2.81 Å resolution, in which we identify multiple pigment-binding α- and β-polypeptides within an LH1 ring. Of the 16 α-polypeptides, we show that six (α1) bind Ca²⁺ along with β1- or β3-polypeptides to form the Ca²⁺-binding sites. This structure differs from that of fully Ca²⁺-bound LH1 from Thermochromatium tepidum, enabling determination of the minimum structural requirements for Ca²⁺-binding. We also identified three amino acids (Trp44, Asp47, and Ile49) in the C-terminal region of the A. tepidum α1-polypeptide that ligate each Ca ion, forming a Ca²⁺-binding WxxDxI motif that is conserved in all Ca²⁺-bound LH1 α-polypeptides from other species with reported structures. The partial Ca²⁺-bound structure further explains the unusual phenotypic properties observed for this bacterium in terms of its Ca²⁺-requirements for thermostability, spectroscopy, and phototrophic growth, and supports the hypothesis that A. tepidum may represent a “transitional” species between mesophilic and thermophilic purple sulfur bacteria. The characteristic arrangement of multiple αβ-polypeptides also suggests a mechanism of molecular recognition in the expression and/or assembly of the LH1 complex that could be regulated through interactions with reaction center subunits.

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

Light-harvesting complex 1 (LH1) of the thermophilic purple sulfur bacterium Thermochromatium tepidum can be expressed in the purple non-sulfur bacterium Rhodobacter sphaeroides and forms a functional RC-LH1 complex with the native Rba. sphaeroides reaction center (Nagashima, K. V. P., et al. Proc. Natl. Acad. Sci. U. S. A. 2017, 114, 10906-10911). Although there is a large uphill energy gap between Tch. tepidum LH1 and the Rba. sphaeroides RC in this chimeric complex, it has been shown that light energy can be transferred, consistent with that seen in the native Rba. sphaeroides RC-LH1 complex. In this study, the contribution of this chimeric complex to growth and photosynthetic energy conversion in the hybrid organism was quantified. The mutant synthesizing this chimeric complex was grown phototrophically under 940 nm light-emitting diode (LED) light preferentially absorbed by Tch. tepidum LH1 and showed faster growth at low intensities of this wavelength than both a mutant strain of Rba. sphaeroides lacking LH2 and a mutant lacking all light-harvesting complexes. When grown with 850 nm LED light, the strain containing the native Rba. sphaeroides LH1-RC grew faster than the chimeric strain. Electron transfer from the RC to the membrane-integrated cytochrome bc₁ complex was also estimated by flash-induced absorption changes in heme b. The rate of ubiquinone transport through the LH1 ring structure in the chimeric strain was virtually the same as that in native Rba. sphaeroides. We conclude that Tch. tepidum LH1 can perform the physiological functions of native LH1 in Rba. sphaeroides.

A comparative look at structural variation among RC-LH1 'Core' complexes present in anoxygenic phototrophic bacteria

All purple photosynthetic bacteria contain RC-LH1 'Core' complexes. The structure of this complex from Rhodobacter sphaeroides, Rhodopseudomonas palustris and Thermochromatium tepidum has been solved using X-ray crystallography. Recently, the application of single particle cryo-EM has revolutionised structural biology and the structure of the RC-LH1 'Core' complex from Blastochloris viridis has been solved using this technique, as well as the complex from the non-purple Chloroflexi species, Roseiflexus castenholzii. It is apparent that these structures are variations on a theme, although with a greater degree of structural diversity within them than previously thought. Furthermore, it has recently been discovered that the only phototrophic representative from the phylum Gemmatimonadetes, Gemmatimonas phototrophica, also contains a RC-LH1 'Core' complex. At present only a low-resolution EM-projection map exists but this shows that the Gemmatimonas phototrophica complex contains a double LH1 ring. This short review compares these different structures and looks at the functional significance of these variations from two main standpoints: energy transfer and quinone exchange.

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

The light-harvesting 1 reaction center (LH1-RC) complex in the purple sulfur bacterium Thiorhodovibrio ( Trv.) strain 970 cells exhibits its LH1 Q _y transition at 973 nm, the lowest-energy Q _y absorption among purple bacteria containing bacteriochlorophyll a (BChl a). Here we characterize the origin of this extremely red-shifted Q _y transition. Growth of Trv. strain 970 did not occur in cultures free of Ca²⁺, and elemental analysis of Ca²⁺-grown cells confirmed that purified Trv. strain 970 LH1-RC complexes contained Ca²⁺. The LH1 Q _y band of Trv. strain 970 was blue-shifted from 959 to 875 nm upon Ca²⁺ depletion, but the original spectral properties were restored upon Ca²⁺ reconstitution, which also occurs with the thermophilic purple bacterium Thermochromatium ( Tch.) tepidum. The amino acid sequences of the LH1 α- and β-polypeptides from Trv. strain 970 closely resemble those of Tch. tepidum; however, Ca²⁺ binding in the Trv. strain 970 LH1-RC occurred more selectively than in Tch. tepidum LH1-RC and with a reduced affinity. Ultraviolet resonance Raman analysis indicated that the number of hydrogen-bonding interactions between BChl a and LH1 proteins of Trv. strain 970 was significantly greater than for Tch. tepidum and that Ca²⁺ was indispensable for maintaining these bonds. Furthermore, perfusion-induced Fourier transform infrared analyses detected Ca²⁺-induced conformational changes in the binding site closely related to the unique spectral properties of Trv. strain 970. Collectively, our results reveal an ecological strategy employed by Trv. strain 970 of integrating Ca²⁺ into its LH1-RC complex to extend its light-harvesting capacity to regions of the near-infrared spectrum unused by other purple bacteria.