ISSN:
1432-2048
Keywords:
Chloroplasts
;
CO2-fixation
;
SH-groups
;
Sulfate incorporation
;
Sulfite incorporation
Source:
Springer Online Journal Archives 1860-2000
Topics:
Biology
Notes:
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 3 2- binding, but decreases that of 35SO 4 2- , 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 4 2- , 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 3 2- 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 3 2- incorporation is indicated.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00388167
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