Solubilization and spectral characteristics of chlorophyll-protein complexes isolated from the thermophilic blue-green alga Synechococcus lividus

Abstract

The thermophilic blue-green alga Synechococcus lividus was grown at 38 and 55°C. The reaction center chlorophyll-protein complexes (CP) of Photosystem (PS I) and PS II, CP a I and CP a II, were isolated by sodium dodecyl sulphate (SDS) polyacrylamide gel electrophoresis at 4°C. SDS solubilization of thylakoids was performed in the temperature range 0–65°C. The low-temperature absorption and fluorescence emission spectral properties of the isolated chlorophyll-protein complexes were analyzed. Only traces of CP a I were solubilized at temperatures below the lipid phase transition temperature. Instead, a minor PS I component, CP a′ I, was obtained that had absorption and fluorescence characteristics similar to those of CP a I. CP a′ I had a slightly lower mobility than CP a I in SDS-polyacrylamide gel electrophoresis. The amount of CP a I in the gel scan profile increased dramatically when solubilization was carried out above the phase transition temperatures, but started to decrease above 60°C. CP a II, on the other hand, could be efficiently extracted even at 0°C and was stable in the scan profile up to extraction temperatures of 30–40°C. Low-temperature absorption and fluorescence emission spectra were typical for CP a I and CP a II and no specific effects of the two growth temperatures on these properties were observed. The phase transition temperature was considered to be critical for the solubilization of CP a I, either because of the difficulties of SDS (especially as it forms micelles at low temperatures) in penetrating the solidified membrane lipids at temperatures below that of the phase transition or because the CP a I monomers of the PS I antennae are so strongly bound to each other that they cannot be dissociated by SDS before thermal agitation has reached a certain level that is achieved above the phase transition temperature. We consider both the difficulties in solubilizing CP a I at sub-transition temperatures and the heat stability of the two complexes as adaptations which enable Synechococcus to grow under extreme high-temperature regimes.