Studies on photosynthetic mechanism of bacteria having Zn-bacteriochlorophyll

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Major results obtained during the term of this project are summarized as follows :1. Reaction center (RC) and the sole antenna complex, LH 1, were isolated from Acidiphilium (A.) rubrum, a representative species in the Zn-bacteriochlorophyll (Zn-BChl) a producing bacteria. The RC preparation contained Zn-BChl a, bacteriopheophytin and spirilloxanthin in a ratio of 4 : 2 : 1 but no Mg-BChl a, indicating that Zn-BChl a is functioning as the special pair and the accessary BChl.2. Genes for the RC and the LH proteins were found to form a cluster similar to the puf-operon of purple photosynthetic bacteria. Amino acid residues for cofactor binding in the purple bacterial RC were all conserved but the L168His residue which is hydrogen bonded to a side chain of one of the special pair BChl. The L168 residue was Glu in A.rubrum. A weaker interaction between the special pair BChls than that in purple bacterial RC was suggested by analysis of absorption and CD spectra which was in good agreement … More with the substitution of L168His to Glu residue.3. Genes for proteins functioning in metal chelation in the BChl biosynthetic pathway were cloned, and the products were shown to have Mg-chelating activity by functional complimentation. The temporary incorporation of Mg atom into precursors of BChl biosynthesis was confirmed by the detection of Mg-protoporphylin IX monomethylester in A.rubrum cells. Thus, Mg in the protoporphyri ring was shown to be substituted by Zn atom at the later step (s) in the BChl biosynthesis pathway.4. The ratio of Zn-BChl a to Mg-BChl a was around 13 : 2 under pH lower than 4, but was decreased at higher pH.The cellular BChl content was increased under starved conditions. Zn^<2+> concentration of μM order was enough for Zn-BChl formation ; Higher concentration did not enhance Zn-BChl production.5. Zn-BChl was found only from species belonging to genus Acidiphilium. An acidophilic BChl-containing bacteria, Acidisphaera rubrifaciens found by this project could synthesize trace amount of Zn-BChl a in addition to Mg-BChl a when mM order of Zn^<2+> was provided.6. Zn-BChl a was shown to be stable under pH conditions higher than 1 , whitle Mg was released from Mg-BChl a below pH 4. The acid-resistence of Zn-BChl a was in good agreement with the extremely acidophilic nature of Acidiphilium. Less