The phytoene formation process is conserved in various C40 carotenogenesis pathways. That is, the head-to-head condensation of two molecules of geranylgeranyl diphosphate (C20PP) is catalyzed by phytoene synthase (CrtB; Lang et al., 1994; Umeno et al., 2005). Phytoene is then sequentially desaturated by phytoene desaturase (CrtI). Different degrees of desaturation form carotenoids with different lengths of conjugated double bonds. Neurosporene with nine conjugated double bonds, lycopene with 11 conjugated double bonds, and didehydrolycopene

with 13 conjugated double bonds are produced, respectively, by three-, four-, and five-step -phytoene desaturations (Sandmann, 2009). These three products of CrtI are very important intermediates Dabrafenib RO4929097 mw leading to different carotenogenesis pathways in different organisms. Thus, controlling and altering the desaturation step number is important in reconstructing carotenogenesis pathways (Garcia-Asua et al., 1998; Umeno et al., 2005). Phytoene desaturase may be divided into two types: the CrtI-type in nonoxygenic bacteria and the Pds/CrtP-type in oxygenic photosynthetic organisms (Sandmann, 2009). In purple photosynthetic bacteria, CrtI generally catalyzes three types

of desaturation in different carotenogenesis pathways. These types include three-step, four-step, and both three- and four-step phytoene desaturations (Takaichi, 2008). In purple nonsulfur alphaproteobacteria Rhodobacter sphaeroides and Rhodobacter capsulatus, CrtI catalyzes the three-step phytoene desaturation to produce neurosporene. The subsequent hydration, desaturation, methylation, and oxygenation steps catalyzed, respectively, by CrtC, CrtD, CrtF, and CrtA enzymes lead to the synthesis of spheroidene and spheroidenone (Armstrong et al., 1989; Lang et al., 1995). In the purple sulfur gammaproteobacterium Thiocapsa roseopersicina, CrtI catalyzes the four-step phytoene desaturation

to produce lycopene. The subsequent hydration, desaturation, and methylation steps catalyzed, respectively, by CrtC, CrtD, and CrtF enzymes lead to the synthesis of spirilloxanthin (Kovacs et al., 2003). In the purple nonsulfur betaproteobacterium Rubrivivax gelatinosus, CrtI catalyzes both three- and four-step phytoene desaturations to produce neurosporene PRKD3 and lycopene, respectively. Spheroidene, hydroxyspheroidene, and small amounts of spirilloxanthin are synthesized after the abovementioned similar modification steps (Harada et al., 2001). Rhodobacter azotoformans is a purple nonsulfur photosynthetic bacterium first reported by (Hiraishi et al., 1995). Differences were found in carbon source utilization and terminal oxidant utilization in anaerobic darkness and 16S rRNA gene sequences between Rba. azotoformans and Rba. sphaeroides (Hiraishi et al., 1996). So far, no report is available about the carotenogenesis gene cluster and related enzymes of Rba. azotoformans.