Molecular and structural insights into neurosporene accumulation in Rhodobacter sphaeroides G1C.

Carotenoids are tetraterpenoid pigments that play important roles in photosynthesis, provide protection from oxidative stress, and facilitate environmental adaptation. The green-colored carotenoid neurosporene, a key intermediate in carotenoid biosynthesis, plays a central role in the production of several other carotenoids; however, neurosporene accumulation is rare in nature. In this study, we investigated the accumulation of neurosporene in Rhodobacter sphaeroides strain G1C, a mutant derivative of wild-type Rba. sphaeroides 2.4.1 obtained by nitrosoguanidine mutagenesis. Whole-genome sequencing of strain G1C identified a nonsense mutation in the crtC gene resulting in enzyme inactivation and disruption of normal carotenoid biosynthesis. Physiological analysis and cryo-EM structural analysis of the major photosynthetic complexes of strain G1C showed that the accumulation of neurosporene did not significantly alter either the structure or functioning of the light-harvesting complexes and had no perceptible effect on photosynthetic growth. While the carotenoid composition of strain G1C shifts from spheroidene to neurosporene, its photocomplexes remain structurally intact and their physiological properties remain largely unchanged. This study provides genomic insights into the importance of CrtC activity to the production of carotenoids in Rba. sphaeroides and highlights the unexpected finding that carotenoid substitution in photocomplexes can occur without significantly disrupting photocomplex function.