Deletion of a B800−850 Light-Harvesting Complex in Rhodospirillum molischianum DSM119 Leads to “Revertants” Expressing a B800−820 Complex: Insights into Pigment Binding

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Abstract
A B800−850 light-harvesting complex (also called LH2) deficient strain of Rhodospirillum molischianum was constructed by replacing a portion of the LH2 gene cluster by a kanamycin resistance gene cartridge. The LH2 deficient strain was characterized spectroscopically and by Southern blot analysis. Surprisingly, pseudorevertants were obtained which express a B800−820 complex which could not be observed in the wild type. This B800−820 complex was isolated and characterized. It consists of an α- and a β-subunit with 56 and 45 amino acid residues, respectively. The amino acid sequences of both subunits are extremely similar to those of the corresponding B800−850 complex. Resonance Raman spectroscopy shows that in the B800−820 complex the two 2-acetylcarbonyl groups of the bacteriochlorophyll a (BChl a) molecules absorbing at 820 nm are free from hydrogen bond interactions, whereas one of the two 2-acetylcarbonyl groups of the pair of BChl a molecules absorbing at 850 nm of the B800−850 complex is involved in hydrogen bonds. These different protein−pigment interactions are due to the replacement of αTrp43 in the B800−850 complex by a Phe in the B800−820 complex. Comparison of the amino acid sequences of the B800−850 and B800−820 complexes of Rs. molischianum and Rhodopseudomonas acidophila reveals a conserved motif comprised of three amino acid residues. Molecular modeling using the known LH2 structure of Rps. acidophila Ac10050 indicates that this motif might be important for the precise structural arrangement of the native complex and fine tuning of its spectroscopic properties.