Possible alternate pathway of bacteriochlorophyll biosynthesis in a mutant of Rhodopseudomonas sphaeroides

A previously uncharacterized bacteriochlorophyll-less mutant (mutant 8) of Rhodopseudomonas sphaeroides has been found to excrete a tetrapyrrole-protein complex into the incubation medium. The structure of the major pigment of the complex was characterized as 2-desacetyl-2-vinylbacteriopheophorbide. The corresponding magnesium derivative does not fit into the currently proposed biosynthetic pathway for bacteriochlorophyll, and thus may indicate the existence of an alternate pathway of bacteriochlorophyll synthesis in this organism. Such an alternate pathway would be possible if reduction from the chlorin to the tetrahydroporphyrin stage can occur either before or after hydration of the 2-vinyl substituent of chlorophyllide a to an alpha-hydroxyethyl group.