Spectroscopic studies of cyanobacterial phycobilisomes lacking core polypeptides

Abstract

Synechococcus sp. PCC 7002 (Agmenellum quadruplicatum PR6) genes encoding two highly conserved phycobilisome core polypeptides, a small linker polypeptide (LC8, apcC) and the allophycocyanin-B alpha-subunit (alpha 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 LC8 polypeptide and the apcD- mutant assembled phycobilisomes lacking alpha 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 degrees 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 alpha 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.