Probing structure–function relationships in early events in photosynthesis using a chimeric photocomplex

Significance Phototrophic bacteria have provided fundamental insight into the biological mechanism of solar energy conversion. Early events in photosynthesis are carried out by the antenna apparatus for light-harvesting (LH) and the reaction center (RC) for charge separation. Here we describe a system for expressing a chimeric LH1-RC complex from two phylogenetically distant phototrophic purple bacteria: the LH1 from Thermochromatium tepidum and the RC from Rhodobacter sphaeroides . This system is exploited to definitively localize Ca 2+ within the Tch. tepidum LH1 complex. The hybrid photocomplexes also provide powerful new tools for probing photosynthetic energy transfer, identifying intrinsic LH1–RC interactions, monitoring altered behavior of carotenoids in a nonnative environment, and linking specific amino acid residues to the specific spectroscopic properties of different phototrophic organisms.