Photosystem I Particles from Sorghum: Isolation and Characterisation of Chlorophyll-Protein Complexes under Differing Growth Light Regimes

Summary

Photosystem I (PS I) particles were isolated from sorghum thylakoids by Triton-X-100 solubilisation. The purified photosystem I particles possessed a chlorophyll/P700 ratio of 180 and a chlorophyll a/b ratio of 10. The polypeptide composition of the isolated PS I complex revealed a dominant 68kD polypeptide, four polypeptides in the range of 18–25kD associated with LHC-I and six minor polypeptides in the range of 8–24kD. The chlorophyll-protein (chl-protein) complexes of PS I were isolated from the purified particles by centrifugation in SDS sucrose density gradients. The complexes were designated as CP I, CP Ia and LHC-I. Sucrose density gradient ultracentrifugation resulted in two chlorophyll protein complexes associated with LHC-I-730 which were identified by their fluorescence emission spectra.

The changes in the structural organisation of PS I complex under varying levels of growth irradiance were investigated. The CP I cpmplex remained stable while the proportion of CP Ia and LHC-I increased under low growth light. The PS I complex exhibited two fluorescence peaks at 730 and 688 nm associated with core antenna of reaction centre and LHC-I respectively. The enhanced F 688 indicates the increased size of LHC-I. Further, the two complexes of LHC-I, namely LHC-I-730 (inner antenna) and LHC-I-685 (peripheral antenna), exhibited marked variations in their fluorescence characteristics. The results suggest the indispensable role of the inner and peripheral antenna of PS I of sorghum in the efficient harvesting of available photon flux density under light limited conditions.