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Articles

Influence of Light Intensity on Reductive Pentose Phosphate Cycle Activity during Photoheterotrophic Growth of Rhodospirillum rubrum¹

^a Postgraduate School of Studies in Biological Sciences, University of Bradford, Bradford, Yorkshire BD7 1DP, England

Light intensity during growth affects the proportion of carbon dioxide fixed by the reductive pentose phosphate cycle relative to that incorporated via C₄ acids in acetate phototrophs of Rhodospirillum rubrum. With cells grown at high light intensity (9000 lux) the specific activities of ribulose-1, 5-diphosphate and propionyl CoA carboxylases were increased compared with cells grown at low light intensity (1500 lux), although pyruvate carboxylase activity was unaltered.

Kinetic experiments with cells assimilating acetate at high light intensity showed that when the cells had been grown at high light intensity there was a rapid incorporation of ¹⁴CO₂ into phosphate esters compared with cells grown at low light intensity and fixing ¹⁴CO₂ while assimilating acetate at low light intensity. The percentage of the total radioactivity present in phosphate esters plotted against time gave a negative slope for high light conditions compared with a positive slope for low light conditions. High light-grown cells assimilating acetate at high light intensity showed the greatest combined rate of ¹⁴CO₂ fixation via the reductive pentose phosphate cycle and C₄ acids, and this corresponded to the shortest mean generation time. When cells were grown at high light intensity and allowed to assimilate ¹⁴CO₂ at high light intensity but in the stationary phase, the pattern of ¹⁴CO₂ fixation resembled that for low light-grown cells assimilating acetate and fixing ¹⁴CO₂ at low light intensity, showing that both acetate assimilation and high light intensity were necessary for the rapid incorporation of ¹⁴CO₂ via the reductive pentose phosphate cycle.

¹ This work was supported by Science Research Council studentship to J.P.