The complete amino acid sequence of the bacteriochlorophyllcbinding polypeptide from chlorosomes of the green photosynthetic bacteriumChloroflexus aurantiacus

The BChlc polypeptide was isolated from chlorosomes of the green bacterium Chloroflexus aurantiacus on Sephadex LH-60. The complete amino acid sequence of this 5.6 kDa polypeptide (51 amino acid residues) was determined. Most probably the 5.6 kDa polypeptide forms an α-helix between Trp 5 and Ile 42 with an asymmetrical (bipolar) distribution of polar amino acid residues along the helix axis: (i) At one side of the α-helix 5 Gln and 2 Asn residues are the possible binding sites for 7 BChlc molecules, (ii) On the other side Ser, Thr, His residues seem to be polypeptide-polypeptide interaction sites within the BChlc-protein complexes. It appears that the BChl-protein complex (chlorosome subunit, 5.2 × 6 nm) composed of 12 5.6 kDa polypeptides corresponds to the 'globular units' found by electron microscopy within the chlorosomes.