MOLECULAR-GENETIC ANALYSIS OF TERMINAL STEPS IN BACTERIOCHLOROPHYLL ABIOSYNTHESIS - CHARACTERIZATION OF A RHODOBACTER-CAPSULATUS STRAIN THAT SYNTHESIZES GERANYLGERANIOL-ESTERIFIED BACTERIOCHLOROPHYLL A

Site-directed mutational analysis of the Rhodobacter capsulatus photos ynthesis gene cluster was undertaken in order to identify and characte rize genetic loci involved in bacteriochlorophyll a biosynthesis. A mu tant in orf304 was shown to accumulate the tetrapyrrole intermediate ' 'bacteriochlorophyllide a'' which is a tetrapyrrole that has a bacteri ochlorophyll a ring structure without the presence of an esterifying a lcohol. A mutant in orf391 is shown to synthesize bacteriochlorophyll a that is esterified with geranylgeraniol rather than the normal phyto l. This latter result provides the first genetic confirmation that est erification of bacteriochlorophyllide a initially involves the additio n of a geranylgeraniol group followed by sequential reduction of the g eranyl geraniol moiety to phytol which is the end product of the pathw ay. An R. capsulatus strain synthesizing geranylgeramiol-esterified ba cteriochlorophyll is shown to exhibit severely impaired photosynthetic growth capability. This is despite our observation that synthesis of geranylgeraniol-esterified bacteriochlorophyll does not affect the ene rgy transfer rate from light harvesting to reaction center complexes n or the electron transfer function as measured by the yield of electron transfer to the primary and secondary quinones, the charge recombinat ion rate from the quinones, and the rate of cytochrome ct oxidation. W e conclude that the observed reduction of the photosynthetic growth ra te observed for R. capsulatus strains that synthesize geranylgeraniol- estefified bacteriochlorophyll is primarily a consequence of the reduc ed steady-state level of the photosystem.