Spectroscopic studies of cyanobacterial phycobilisomes lacking core polypeptides

Synechococcus sp. PCC 7002 (Agmenellum quadruplicatum PR6) genes encoding two highly conserved phycobilisome core polypeptides, a small linker polypeptide (L_C⁸, apcC) and the allophycocyanin-B α-subunit (α^APB, apcD), respectively, were interrupted by insertion of restriction fragments carrying the neomycin phosphotransferase gene of Tn5. The interrupted genes were used to transform Synechococcus sp. PCC 7002 to kanamycin resistance. The apcC^- mutant assembled phycobilisomes lacking the L_C⁸ polypeptide and the apcD^- mutant assembled phycobilisomes lacking α^APB. No other differences between the compositions of the mutant and wild-type phycobilisomes were detected. The apcC^- strain grew about 25% more slowly than the wild-type, and its phycobilisomes dissociated more rapidly in 0.33 M Na/K-PO₄ (pH 8.0) or in 0.75 M Na/K-PO₄ at pH 8.0, at 40°C, than did those of the wild-type. The phycobilisomes of this mutant were indistinguishable from those of the wild-type with respect to absorption and circular dichroism spectra, as well as time-resolved fluorescence emission. Steady-state emission spectra indicate a small decrease in long wavelength (680 nm) emission from the apcC^- phycobilisomes and a complementary increase in shorter wavelength (665 nm) emission, relative to wild-type phycobilisomes. Strain apcD^- phycobilisomes appear to be functionally indistinguishable from those of the wild-type, in spite of the absence of the two α^APB subunits which bear terminal acceptor bilins. The only spectroscopic difference was seen in the steady-state fluorescence emission, for which the emission of the mutant was about 15% higher than that of the wild-type and was slightly blue-shifted. A phenotype has yet to be found for the apcD^- mutation.