Low Concentrations of Tetracycline Enhance the Photophosphorylation and P/O Ratio of Chloroplasts.

This study investigated the effects of tetracycline on photophosphorylation, electron transport and P/O ratio of spinach chloroplasts. When chloroplast preparations were treated with low concentrations of tetracycline, non-cyclic and cyclic photophosphorylation activities increased, electron transport rates and P/O ratios improved, chloroplast ms-DLE also improved, and the Mg(2+)-ATPase activity of CF1 increased in comparison to the control. These results indicate that spinach chloroplasts are sensitive to tetracycline. Next, we used the fluorescence emission spectra of CF1 to examine the possible binding sites for tetracycline. The fluorescence emission spectra of CF1 treated with glutaraldehyde, NEM and TNBS, which interact with CF1 across its whole structure, at the γ subunit and at the β subunit, respectively, were compared with that of control CF1. The peak sites of the various fluorescence emission spectra were the same, but the peak values for CF1 treated with glutaraldehyde, NEM and TNBS were lower than that of control CF1. The peak value of CF1 treated with 50 µM tetracycline was very similar to that of CF1 treated with NEM. The above results indicate that the acting site of tetracycline may be at or near the γ subunit of CF1, and allows the creation of a model in which tetracycline binding strengthens the subunit interactions of ATP synthase, enlarges the proton motive force across the thylakoid membrane, and allows the excess proton motive force to increase ATP formation and improve the P/O ratio.