An association between photorespiration and protein catabolism: Studies with Chlamydomonas

Work demonstrating the operation of a photorespiratory N cycle in Chlamydomonas is described. NH3 release by this process is light dependent, sensitive to changes in pO2 and pCO2, and abolished by a photosystem II inhibitor. Evidence is presented which shows that this NH3 derives its N from protein rather than from freshly synthesised glutamate. Protein turnover is shown to provide amino-N at a rate sufficient to account for the highest photorespiratory N excretion observed suggesting that changes in excretion can be accounted for by increased catabolism of normally recirculating amino acids. It is equally possible however that a direct link between photorespiration and protein turnover exists, increased NH3 excretion resulting from enhanced protein turnover. The data suggest that if similar mechanisms operate in higher plants, previous estimates of the amount of N recycled in photorespiration may have been too high.