Structural differences in chlorosomes fromChloroflexus aurantiacusgrown under different conditions support the BChlc-binding function of the 5.7 kDa polypeptide

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Abstract
Structurally different chlorosomes were isolated from the green photosynthetic bacterium Chloroflexus aurantiacus grown under different conditions. They were analysed with respect to variable pigment-protein stoichiometries in view of the presumed BChI c-binding function of the 5.7 kDa chlorosome polypeptide. Under high-light conditions on substrate-limited growth medium the pigment-protein ratio of isolated chlorosomes was several times lower than under low-light conditions on complex medium. Proteolytic degradation of the 5.7 kDa polypeptide in high-light chlorosomes led to a 60% decrease of the absorbance at 740 nm. The CD spectrum of high-light chlorosomes exhibited a sixfold lower relative intensity at 740 nm (ΔA/A 740) than low-light chlorosomes, but it showed a fivefold increase in intensity upon degradation of the 5.7 kDa polypeptide compared to a twofold increase in low-light chlorosomes. It seems probable that BChl c in the chlorosomes is present as oligomers bound to the 5.7 kDa polypeptide. Our data suggest further that compared to low-light chlorosomes smaller oligomers or single BChl c molecules are bound to the 5.7 kDa polypeptide in high-light chlorosomes resulting in lower rotational strength.

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