Control of ³⁵SO ^2-₄ and ³⁵SO ^2-₃ incorporation into spinach chloroplasts during photosynthetic CO₂ fixation

Abstract

In addition to membrane translocation, measured in the dark, it was found that pre-illumination of the chloroplasts resulted in an enhancement of sulfate uptake by 25% and of sulfite uptake by 55% as soon as the concentration of the ion in the incubation medium exceeded 2 mmol l^-1. This amount which is additionally taken up after pre-illumination is less readily exchanged for other ions. Kinetics of the uptake in relation to pre-illumination time and to light intensity closely parallel those of titration of SH-groups by 5,5′-dithiobis (2-nitrobenzoic acid). As a consequence, 10^-6 mol l^-1 DCMU completely inhibits the light triggered increase of uptake of both ions. Uncoupling with 10^-6 mol l^-1 CCCP increases the light induced ³⁵SO ^2-₃ binding, but decreases that of ³⁵SO ^2-₄ , demonstrating the need of ATP formation to initiate sulfate reduction. Rates of uptake, measured at different intensities of pre-illumination under nitrogen or in the presence of bicarbonate, suggest that the presence of a carbon skeleton increases the binding rate for both ions. With respect to ³⁵SO ^2-₄ , the data further indicate a rate limiting step (ATP sulfurylase or adenosine 5′-phosphosulfate sulfotransferase) which is activated by light, thus representing a control step to harmonize the rate of CO₂ fixation and of sulfate incorporation. On the contrary, ³⁵SO ^2-₃ is directly bound in relation to the amount of SH-groups, which in turn are created by the photosynthetic electron transport, resulting in Car-S-SO ^-₃ . Since the formation of SH-groups is maximal already at low light intensities, no effective control step for SO ^2-₃ incorporation is indicated.