Light-dependent synthesis of glutamine in pea-chloroplast preparations.

Isolated-chloroplast preparations obtained from Pisum sativum (L.) plants synthesized L-[(14)C]glutamine from L-[(14)C]glutamate in the light. In the dark appreciable synthesis of glutamine occurred only in the presence of added ATP. Photoconversion of glutamate to glutamine was inhibited by millimolar concentrations of ammonium or nitrite, and by DCMU. The inhibition due to DCMU, NO2 (-), or NH4 (+) was markedly relieved by ATP. ATP-dependent synthesis of glutamine in the dark was not inhibited by ammonium or nitrite ions.The data suggest that ATP necessary to support chloroplastic glutamine synthesis may be derived from noncyclic or pseudocyclic phosphorylation. Since high ammonia concentrations appear to inhibit chloroplastic glutamine synthesis by uncoupling photophosphorylation, it is possible that ATP synthesized outside the chloroplast may be required to drive the rapid synthesis of glutamine that occurs in leaves subjected to toxic levels of ammonia.