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. //
