Altered monovinyl and divinyl protochlorophyllide pools in bchJ mutants of Rhodobacter capsulatus. Possible monovinyl substrate discrimination of light-independent protochlorophyllide reductase.

In land plants in particular, it has been well established that chlorophyll intermediates, Mg-protoporphyrin, Mg-protoporphyrin monomethylester, protochlorophyllide, and chlorophyllide occur as monovinyl and divinyl forms. The pool of monovinyl and divinyl intermediates differ according to species, age of tissue, and light regime. In this study, we investigated the monovinyl and divinyl characteristics of protochlorophyllide and chlorophyllide in the purple non-sulfur photosynthetic eubacterium Rhodobacter capsulatus. Our results indicate that mutations in genes known to completely block the reduction of protochlorophyllide to chlorophyllide (such as bchN, bchB, and bchL mutants), accumulate a pool of monovinyl and divinyl forms of protochlorophyllide just as observed in plants. However, we also observed that directed insertion and deletion mutations in bchJ, a gene located in the photosynthesis gene cluster, affected the ratio of monovinyl and divinyl protochlorophyllide. Specifically, bchJ-disrupted strains accumulate reduced levels of bacteriochlorophyll concomitant with the accumulation of divinyl protochlorophyllide. Mutants of bchJ in combination with a second mutation in bchL still produce a mixed pool of monovinyl and divinyl protochlorophyllide; however, the ratio of monovinyl to divinyl protochlorophyllide is skewed in favor of divinyl protochlorophyllide. These results thus identify bchJ as the first sequenced gene that affects the divinyl to monovinyl ratio of photopigment intermediates in any photosynthetic organism. In addition, the results of our study also suggest that light-independent protochlorophyllide reductase is discriminatory for a monovinyl substrate.