Regulation of Tetrapyrrole Synthesis in Rhodobacter capsulatus

光合作用细菌使用单一分支途径合成所有四个四吡咯的最终产物：亚铁血红素、细菌叶绿素、维生素B-12和四氢亚铁血红素。这一分支途径之所以如此有趣，是因为这四种产物在细胞新陈代谢中扮演着如此不同的角色，所需的量也截然不同。这项研究的最终目的是了解光合细菌胶囊红杆菌如何协调四个四吡咯产物的合成。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的过程。因此，细菌叶绿素的合成受细胞中存在的PufQ蛋白数量的调节。像PufQ这样的载体蛋白在调节共同的四吡咯途径中的作用正在被确定，该途径导致血红素和细菌叶绿素。当从被囊圆环菌中排出共比例卟啉时，它与一个32 kDa的蛋白络合，该蛋白太大而不能成为PufQ。该蛋白质正在进行部分测序，使用寡核苷酸探针克隆该基因，并构建了一个缺失该蛋白质的突变体。根据该突变体的表型，可以确定该蛋白是否对血红素合成、细菌叶绿素合成或两者都是必需的。正在分离其他HEM突变体，以便确定哪个四吡咯中间体是第一个与载体蛋白结合的中间体。可以说，导致产生叶绿素和血红素等四个吡咯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是如何调控的知之甚少。光合作用细菌胶囊红假单胞菌提供了一个理想的模式系统，因为它合成了所有四个四吡咯的最终产物，并且易于从遗传学和生物化学的角度进行研究。这项研究的目的是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. //