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Publication Abstract from PubMed

The genome of the mildly thermophilic hot spring purple sulfur bacterium, Allochromatium (Alc.) tepidum, contains a multigene pufBA family that encodes a series of alpha- and beta-polypeptides, collectively forming a heterogeneous light-harvesting 1 (LH1) complex. The Alc. tepidum LH1, therefore, offers a unique model for studying an intermediate phenotype between phototrophic thermophilic and mesophilic bacteria, particularly regarding their LH1 Qy transition and moderately enhanced thermal stability. Of the 16 alpha-polypeptides in the Alc. tepidum LH1, six alpha1 bind Ca(2+) to connect with beta1- or beta3-polypeptides in specific Ca(2+)-binding sites. Here, we use the purple bacterium Rhodospirillum rubrum strain H2 as a host to express Ca(2+)-bound and Ca(2+)-free Alc. tepidum LH1-only complexes composed of alpha- and beta-polypeptides that either contain or lack the calcium-binding motif WxxDxI; purified preparations of each complex were then used to test how Ca(2+) affects their thermostability and spectral features. The cryo-EM structures of both complexes were closed circular rings consisting of 14 alphabeta-polypeptides. The Q(y) absorption maximum of Ca(2+)-bound LH1 (alpha1/beta1 and alpha1/beta3) was at 894 nm, while that of Ca(2+)-free (alpha2/beta1) was at 888 nm, indicating that Ca(2+) imparts a Q(y) transition of 6 nm. Crucially for the ecological success of Alc. tepidum, Ca(2+)-bound LH1 complexes were more thermostable than Ca(2+)-free complexes, indicating that calcium plays at least two major roles in photosynthesis by Alc. tepidum-improving photocomplex stability and modifying its spectrum.

Probing the Dual Role of Ca(2+) in the Allochromatium tepidum LH1-RC Complex by Constructing and Analyzing Ca(2+)-Bound and Ca(2+)-Free LH1 Complexes.,Zou MJ, Sun S, Wang GL, Yan YH, Ji W, Wang-Otomo ZY, Madigan MT, Yu LJ Biomolecules. 2025 Jan 14;15(1):124. doi: 10.3390/biom15010124. PMID:39858518^[1]

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.