Studies on photosynthetic mechanism of bacteria having Zn-bacteriochlorophyll

本研究取得的主要成果如下：1.从嗜酸菌Acidiphilium（A.）红色细菌（rubrum）是锌细菌叶绿素a（Zn-BChl）产生菌中的代表菌种。RC制剂以4：2：1的比例含有Zn-BChl a、细菌脱镁叶绿素和藻黄素，但不含Mg-BChl a，表明Zn-BChl a起特殊对和辅助BChl. 2的作用。发现RC和LH蛋白的基因形成类似于紫色光合细菌的puf操纵子的簇。在紫色细菌RC中用于辅因子结合的氨基酸残基都是保守的，除了L168 His残基，其与特殊对BChl之一的侧链氢键合。红色曲霉的L168残基为Glu。吸收光谱和圆二色光谱分析表明，BChls与紫色细菌RC之间的相互作用较弱，这一结果与紫细菌RC之间的相互作用一致 ...更多信息 其中L168 His被替换为Glu残基。克隆了在BChl生物合成途径中的金属螯合中起作用的蛋白质的基因，并且通过功能互补显示产物具有Mg螯合活性。Mg原子暂时掺入BChl生物合成的前体中通过检测红色曲霉细胞中的Mg-原卟啉IX单甲酯来证实。因此，在BChl生物合成途径的后期，原卟啉环中的Mg被Zn原子取代. Zn-BChl a与Mg-BChl a的比值在pH低于4时约为13：2，在pH高于4时降低，饥饿条件下BChl含量增加。Zn ~（2+）浓度在μM量级时就足以促进Zn-BChl的生成，较高浓度对Zn-BChl的生成没有促进作用. Zn-BChl仅在属于嗜酸菌属的物种中发现。本课题发现的一株嗜酸BChl菌--红色嗜酸菌（Acidisphaera rubrifaciens），当Zn^（2+）浓度为mM时，除Mg-BChl a外，还能合成微量的Zn-BChl a. Zn-BChl a在高于1的pH条件下显示稳定，而Mg在低于pH 4时从Mg-BChl a释放。Zn-BChl a的耐酸性与嗜酸菌的极端嗜酸性一致。少

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