Structural integrity and near-infrared absorption of the LH1 complex of Thermochromatium tepidum: Influence from the C-terminal lysine residues of LH1 α-polypeptide.

The light-harvesting complex 1-reaction center (LH1-RC) photosystem of the thermophilic purple sulfur bacterium Thermochromatium (Tch.) tepidum exhibits a near-infrared LH1-Q absorption band at 915 nm as regulated by binding calcium ions (Ca). To further explore the possible involvement of the C-terminal lysine residues of the LH1 α-polypeptide, we have genetically engineered a Rhodospirillum rubrum mutant strain to yield the site-directed modifications of the terminal α-Lys60 and α-Lys61 residues of Tch. tepidum LH1 α-polypeptide. Four of the LH1 mutants exhibit a subtle blue shift of 3 nm upon deletion or substitution of the lysine residues, however, they display over 40 nm blue shifts upon Ca removal by ethylene diamine tetraacetic acid (EDTA) treatment. Spectral properties of native Tch. tepidum LH1-RC, the LH1-only, and the mutant LH1-only complexes are compared on a structural basis, which allows us to conclude that the C-terminal lysine residues and the Ca binding synergistically affect the structural integrity and the LH1-Q spectral shift. This work demonstrates a methodology for the genetic manipulation of photosynthetic proteins lacking mutagenesis information, and may shed light on understanding the detailed structural factors involved in tuning the LH1-Q absorption.