Phycobilisome composition and possible relationship to reaction centers

Abstract

The photosynthetic apparatus was studied in Anacystis nidulans wild type and in a spontaneous pigment mutant 85Y which had improved growth in far-red light (>650 nm). Two phycobiliproteins, C-phycocyanin ( λ max 625) and allophycocyanin ( λ max 650), were present in a molar ratio of ~3:1 in the wild type and ~0.4:1 in the mutant. Phycobilisomes of wild type cells were larger (57 × 30 nm) than those of the mutant 85Y (28 × 15 nm). In the mutant they seemed to consist primarily of the allophycocyanin core. Fluorescence emission maxima of wild type and mutant 85Y phycobilisomes were at 680 nm (23 °C) and 685 nm (−196 °C). Excitation maxima of phycobilisomes were at 630 and 650 nm for the wild type and the mutant 85Y, respectively. The phycobilisomes of wild type cells whether grown in white or far-red light had the same size and pigment composition. A typical wild type cell in white light had a thylakoid area of 22.8 μm 2, but in far-red light the area was reduced to 13.5 μm 2, which was close to that of 85Y at 13.6 μm 2. Chlorophyll molecules per cell decreased in far-red light from 1.1 × 10 7 in wild type (white light) to 4.5 × 10 6 in mutant 85Y (far-red). The number of phycobilisomes per cell (approx 2 × 10 4), calculated from the phycobiliprotein content and phycobilisome size, was about the same in wild type (white light) and mutant 85Y (far-red light), but the number of phycobilisomes per unit area of thylakoid was significantly greater in mutant 85Y than in wild type. The present results suggest that the phycobilisomes are linked with reaction centers and that the PSII complement (photosystem II and phycobilisome) was fully maintained in far-red light.