Light Harvesting in Anacystis nidulans Studied in Pigment Mutants

Spontaneous pigment mutants of Anacystis nidulans were self-selected for improved growth in far red light (> 650 nanometers). Questions were asked about those features of the light-harvesting mechanism which altered to give the mutants improved photosynthetic performance in far red. Answers were sought by comparing pigment and reaction center concentrations for the parent and six mutants grown in gold fluorescent and in far red light. Three significant results emerged. The ratio of reaction centers for photoreactions I and II (RC1/RC2) varied by a value of about 2.1 for all clones grown in gold and a value of about 1.1 for all clones grown in far red. Alteration of the ratio was not evident in any of the mutants.Phycobilisome alterations were evident as decreased phycocyanin content in all mutants. In three mutants, allophycocyanin became the major remaining phycobilisome component. Action spectra for photoreactions I and II allowed estimates of chlorophylls serving each of the two reaction centers. Ratios of chlorophylls to reaction centers within each photosystem were chlorophyll I/RC1 = 118 +/- 11 and chlorophyll II/RC2 = 52 +/- 9 for all seven clones grown in both gold and far red light. Remarkable constancy of these ratios, in spite of wide variation in cell material, supports an hypothesis that in A. nidulans there are two chlorophyll proteins, each bearing a reaction center and chlorophylls in fixed ratio.