Effect of oxygen on photosynthesis and biosynthesis of glycolate in photoheterotrophically grown cells of Rhodospirillum rubrum

Photosynthetic CO₂ fixation was studied using cells of Rhodospirillumrubrum grown heterotrophically on malate or butyrate. Ratesof CO₂ fixation were higher in the malategrown cells than inthe butyrate-grown bacteria but ribulosebisphosphate (RUP₂)carboxylase/oxygenase activities were higher in the extractsprepared from the butyrate-grown bacteria. The photosyntheticCO₂ fixation in the butyrate-grown R. rubrum cells was inhibitedby KCN, and the inhibitory effect of O₂ on CO₂ fixation wasreversed when cells were returned to an N₂ atmosphere. In themalate-grown cells, photosynthetic CO₂ fixation was insensitiveto KCN and the inhibitory effect exerted by O₂ was practicallyirreversible. ¹⁴CO₂ was not incorporated into glycolate by either malate-or butyrate-grown cells in an N₂ atmosphere, but small amountsof labeled glycolate were found in malate- and butyrate-growncells in air or 100% O₂. Glycolate excreted by these cells in100% O₂ was measured colorimetrically and its identity establishedby mass spectrometry. When the O₂ atmosphere was labeled with¹⁸O₂, only one of the carboxyl oxygens of the excreted glycolatewas labeled, and the enrichment of ¹⁸O in this carboxyl oxygenrelative to the ¹⁸O₂ provided was greater than 80%. These studiesshow that significant glycolate production by R. rubrum onlyoccurs in the presence of O₂ and that in both malateand butyrate-growncells, the glycolate so formed is presumably produced via RuP₂oxygenase. ¹ Paper No. 46 in the series "Structure and Function of ChloroplastProteins", and research supported in part by research grantsfrom the Japanese Ministry of Education (No. 211113), the TorayScience Foundation (Tokyo), and the Nissan Science Foundation(Tokyo). (Received August 19, 1978; )