On the long-wavelength spectral forms of chlorophyll a in Photosystem I: Spectroscopic and immunological investigations on a greening mutant of the green alga Scenedesmus obliquus

Abstract

The origin of the long-wavelength chlorophyll (Chl) absorption (λ_peak > 680 nm) and fluorescence emission (λ_peak > 685 nm) has been investigated on Scenedesmus mutants (C-2A′-series, lacking the ability to synthesize chlorophyll in the dark) grown at 0.3 (LL), 10 (ML) and 240 µE s⁻¹ m⁻²(HL). LL cells are arrested in an early greening state; consequently, ‘Chl availability’ determines the phenotype. LL thylakoids are totally lacking long-wavelength Chl; nonetheless, PS I and PS II are fully functional. Gel electrophoresis and Western blots indicate that four out of seven resolved LHC polypeptides seem to require a high Chl availability for assembly of functional chlorophyll-protein complexes. The PS I core-complex of ML and HL thylakoids contains long-wavelength chlorophylls, but in the PS I core-complex of LL thylakoids these pigments are lacking. We conclude that long-wavelength pigments are only present in the PS I core in the case of high Chl availability. The following hypothesis is discussed: Chl availability determines not only the LHC polypeptide pattern, but also the number of bound Chl molecules per individual pigment-protein complex. Chl-binding at non-obligatory, peripheral sites of the pigment-protein complex results in long-wavelength Chl. In the case of low Chl availability, these sites are not occupied and, therefore, the long-wavelength Chl is absent.