A heterogeneous tag-attachment to the homodimeric type 1 photosynthetic reaction center core protein in the green sulfur bacterium Chlorobaculum tepidum

The 6xHis-tag-pscA gene, which was genetically engineered to express N-terminally histidine (His)-tagged PscA, was inserted into a coding region of the recA gene in the green sulfur bacterium Chlorobaculum tepidum (C. tepidum). Although the inactivation of the recA gene strongly suppressed a homologous recombination in C. tepidum genomic DNA, the mutant grew well under normal photosynthetic conditions. The His-tagged reaction center (RC) complex could be obtained simply by Ni2+-affinity chromatography after detergent solubilization of chlorosome-containing membranes. The complex consisted of three subunits, PscA, PscB, and PscC, in addition to the Fenna–Matthews–Olson protein, but there was no PscD. Low-temperature EPR spectroscopic studies in combination with transient absorption measurements indicated that the complex contained all intrinsic electron transfer cofactors as detected in the wild-type strain. Furthermore, the LC/MS/MS analysis revealed that the core protein consisted of a mixture of a His-/His-tagged PscA homodimer and a non-/His-tagged PscA heterodimer. The development of the pscA gene duplication method presented here, thus, enables not only a quick and large-scale preparation of the RC complex from C. tepidum but also site-directed mutagenesis experiments on the artificially incorporated 6xHis-tag-pscA gene itself, since the expression of the authentic PscA/PscA homodimeric RC complex could complement any defect in mutated His-tagged PscA. This method would provide an invaluable tool for structural and functional analyses of the homodimeric type 1 RC complex.

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► Homodimeric type 1 RC in green sulfur bacteria consists of two identical PscA core subunits.
► Insertion of the His-pscA gene into the recA region.
► The His-tagged RC complex has no PscD subunit but shows intrinsic electron transfer activity.
► Evidence of the heterogeneous tag-attachment by LC/MS/MS analysis.
► A site-directed mutagenesis on the His-pscA gene would be possible.