Some physical properties of an unusual C-phycocyanin isolated from a photosynthetic thermophile

Abstract

A cyanobacterium, which is a photosynthetic thermophile, has been grown in the laboratory at 66–70°C. This organism, Synechococcus lividus (SyI), was isolated from a hot spring in Yellowstone National Park where it grows at 68–73°C. The biliprotein, C-phycocyanin, has been purified, and some of its physical properties studied. When carefully prepared, the protein is obtained in a homogeneous form at about 600000 molecular weight. Its visible absorption spectrum has a maximum at 608 nm, which is blue shifted to a higher energy maximum than any other known C-phycocyanin. Studies are presented showing that the unique spectrum is not a function of protein aggregation, chemically changed chromophores, number of chromophores, linker polypeptides, temperature of measurement, or of its thermophilic origin. To produce this spectrum, one or more of the chromophores must be affected by apoprotein differently that it is affected in other proteins. The circular dichroism spectra of the protein have been studied, and the aggregation was monitored by fluorescence polarization and gel-filtration column chromatography. A method to prepare SyI monomers was developed. Rods, monomers, trimers, and hexamers were all shown to have blue-shifted absorption maxima. Three other C-phycocyanins — one from a mesophilic and two thermophilic cyanobacteria — were compared with the S. lividus (SyI) protein, which was found to be the most thermally stable and most resistent to dissociation.