Regulation of Tetrapyrrole Synthesis in Rhodobacter capsulatus

光合细菌使用单一分支途径合成所有四吡咯最终产物：血红素，细菌叶绿素，维生素B-12和siroheme。这种分支通路的有趣之处在于，这四种产物在细胞代谢中扮演着不同的角色，所需的量也大不相同。本研究的最终目的是了解光合细菌荚膜红杆菌如何协调四吡咯产物的合成。最近的证据表明，PufQ蛋白与一些四吡咯中间体结合，并参与其转化为细菌叶绿素。因此，细菌叶绿素的合成受到细胞中PufQ蛋白含量的调节。载体蛋白如PufQ在调节导致血红素和细菌叶绿素的常见四吡咯途径中的作用正在被确定。当coproporphyrin从荚膜菌中排出时，它与一个32kDa的蛋白络合，该蛋白太大而不可能是PufQ。目前正在对该蛋白进行部分测序，利用寡核苷酸探针克隆该基因，并构建一个缺乏该蛋白的突变体。该突变体的表型允许确定该蛋白质是否需要血红素合成，细菌叶绿素合成，或两者都需要。其他的突变体正在被分离出来，这样就可以确定哪种四吡咯中间体首先结合载体蛋白。可以认为，导致叶绿素和血红素等四吡啶产生的生物合成途径是自然界中最重要的途径之一。目前，人们对这一复杂途径的调控机制知之甚少。光合细菌荚膜红杆菌（Rhodobacter capsulatus）提供了一个理想的模型系统，因为它可以合成所有四吡咯最终产物，并且可以进行遗传和生化研究。这项研究旨在了解如何确保这些化合物的充足而平衡的供应。//

Photosynthetic bacteria use a single branched pathway to synthesize all four tetrapyrrole endproducts: heme, bacteriochlorophyll, vitamin B-12, and siroheme. What makes this branched pathway so interesting is that the four products play such diverse roles in cellular metabolism and are required in vastly different amounts. The ultimate aim of this research is to understand how the photosynthetic bacterium Rhodobacter capsulatus coordinates the synthesis of the four tetrapyrrole products. Recent evidence suggests that the PufQ protein binds to some tetrapyrrole intermediate and is involved in its conversion to bacteriochlorophyll. Synthesis of bacteriochlorophyll is therefore regulated by the amount of PufQ protein present in the cell. The role of carrier proteins such as PufQ in the regulation of the common tetrapyrrole pathway that leads to heme and bacteriochlorophyll is being determined. When coproporphyrin is excreted from R. capsulatus it is complexed with a 32kDa protein which is too large to be PufQ. This protein is being partially sequenced, the gene cloned using oligonucleotide probes, and a mutant constructed that lacks the protein. The phenotype of this mutant allows for a determination of whether this protein is required for heme synthesis, bacteriochlorophyll synthesis, or both. Other hem mutants are being isolated so that it can be determined which tetrapyrrole intermediate is the first to bind a carrier protein. It can be argued that the biosynthetic pathway leading to the production of tetrapyrroles such as chlorophyll and heme is one of the most important pathways in nature. Currently, very little is known about how this complex pathway is regulated. The photosynthetic bacterium Rhodobacter capsulatus provides an ideal model system, in that it synthesizes all four tetrapyrrole endproducts and is amenable to study both genetically and biochemically. This research aims at an understanding of how an adequate yet balanced supply of these compounds is ensured. //