Isolation of a Highly Active PS II Preparation from Wheat Triticum Aestivum and the Chemical Modification of Tyrosine Residues with 4-chloro-7-nitrobenzofurazan (NBD-C1)

Summary

A highly active photosystem II (PS II) preparation was obtained from 8-day old laboratory grown wheat seedlings. Oxygen evolving activity of the PS II preparations was 954 μmoles O₂/mg Chl/hr. The SDS-PAGE showed the presence of 34, 32, 29, 18, 14, 10, 9 and 6 kD proteins. The LHC having M_r = 29 kD was depleted from PS II during isolation as shown by gel analysis. The fluorescence emission spectrum of the LHC obtained after β-octylglucoside treatment had peaks at 683, 693 and 735 nm at 77 K, however, the PS II preparation showed only one emission peak at 685 nm at 77 K. PS II particles from spinach showed slight differences in oxygen evolving activity and relative intensities of bands of different polypeptides in the gel as compared to PS II from wheat.

The 4-chloro-7-nitrobenzofurazan (NBD-C1) modified PS II preparations showed 45 to 55 per cent inhibition in oxygen evolving activity as compared to control. NBD-C1 modified tyrosyl groups of PS II particles also caused significant decrease in F_m and F_v and in light-induced absorbance changes at 680 nm. F_o level was also decreased by this modifier. Dithiothreitol could reverse the inhibition of fluorescence kinetics in NBD-C1 treated PS II particles. The decrease in oxygen evolving activity, light-induced absorbance change at 680 nm and F_v would suggest that treatment of PS II particles with NBD-C1 resulted in the modification of the tyrosine residues of polypeptide(s) involved in electron transport from water to P 680.