A dual role for Ca2+ in expanding the spectral diversity and stability of LH1-RC 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 Qy transition at 973 nm, the lowest-energy Qy absorption among purple bacteria containing bacteriochlorophyll a (BChl a). Here we characterize the origin of this extremely red-shifted Qy 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 Qy 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 ...