Site-directed mutational analysis of the Rhodobacter capsulatus photosynthesis gene cluster was undertaken in order to identify and characterize genetic loci involved in bacteriochlorophyll a biosynthesis. A mutant in orf304 was shown to accumulate the tetrapyrrole intermediate “bacteriochlorophyllide a” which is a tetrapyrrole that has a bacteriochlorophyll a ring structure without the presence of an esterifying alcohol. A mutant in orf391 is shown to synthesize bacteriochlorophyll a that is esterified with geranylgeraniol rather than the normal phytol. This latter result provides the first genetic confirmation that esterification of bacteriochlorophyllide a initially involves the addition of a geranylgeraniol group followed by sequential reduction of the geranylgeraniol moiety to phytol which is the end product of the pathway. An R. capsulatus strain synthesizing geranylgeraniol-esterified bacteriochlorophyll is shown to exhibit severely impaired photosynthetic growth capability. This is despite our observation that synthesis of geranylgeraniol-esterified bacteriochlorophyll does not affect the energy transfer rate from light harvesting to reaction center complexes nor the electron transfer function as measured by the yield of electron transfer to the primary and secondary quinones, the charge recombination rate from the quinones, and the rate of cytochrome c2 oxidation. We conclude that the observed reduction of the photosynthetic growth rate observed for R. capsulatus strains that synthesize geranylgeraniol-esterified bacteriochlorophyll is primarily a consequence of the reduced steady-state level of the photosystem.
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