Effect of carotenoid biosynthesis inhibition on the chlorosome organization in Chlorobium phaeobacteroides strain CL1401

We have studied the effect of the absence of carotenoids on the organization of bacteriochlorophylls (BChls) in chlorosomes of Chlorobium (Chl.) phaeobacteroides strain CL1401. Carotenoid-depleted chlorosomes were obtained by means of 2-hydroxybiphenyl-supplemented cultures. In the presence of the inhibitor, isorenieratene (Isr) and β-Isr biosynthesis were inhibited to more than 95%, leading to an accumulation of the colorless precursor phytoene inside the chlorosomes. In addition, there was a 30-40% decrease in the baseplate BChl a content. The absorption spectrum of the carotenoid-depleted chlorosomes showed a 10 nm blue shift in the BChl e Q_y absorption peak. Under reducing conditions, a decrease in the BChl a/BChl e fluorescence emission ratio was observed in carotenoid-depleted chlorosomes relative to that in control chlorosomes, caused mainly by the decrease in the BChl a content. The steady-state fluorescence emission anisotropy in the BChl e region dropped from ∼0.24 for native chlorosomes to ∼0.14 for carotenoid-depleted ones, indicating reorganization of BChl e. The circular dichroism (CD) signal of the carotenoid-depleted chlorosomes was increased two times in the BChl e Q_y region. A simple model based on the structure proposed was used to explain the observed effects. Carotenoids might affect the angle between the direction of the BChl e Q_y transition and the axis of the rod. The orientation of BChl a in the baseplate remains unchanged in carotenoid-depleted chlorosomes, although there is a partial loss of BChl a as a consequence of a decrease in the baseplate size. The carotenoids are most likely rather close to the BChls and appear to be important for the aggregate structure in Chl. phaeobacteroides.