SERINE SYNTHESIS IN CYANOBACTERIA BY A NONPHOTORESPIRATORY PATHWAY

The pathway of serine synthesis was investigated in six species of cya nobacteria: Anabaena cylindrica, A. variabilis, A. flos-aquae, Coccoch loris peniocystis, Gleocapsa alpicola and Phormidium molle. Activity o f the enzymes phosphoglycerate (PGA) dehydrogenase (EC 1.1.1.95), phos phoserine transaminase (EC 2.6.1.52) and phosphoserine phosphatase (EC 3.1.3.3) was detected in vitro in all species, but no PGA phosphatase (EC 3.1.3.20) or hydroxypyruvate reductase (EC 1.1.1.26) activity cou ld be detected. Metabolism of [1-C-14]PGA by cell-free extracts of C. peniocystis resulted in the labelling of serine, alanine and aspartate , which represented 84, 11 and 4% of the labelled amino acid pool, res pectively. Labelled serine isolated from these experiments was 100% ca rboxyl-labelled indicating that it was formed directly from PGA. The l abelling of serine was markedly reduced by 5 mM phosphoserine. These i n vitro findings indicate that cyanobacteria are capable of synthesizi ng serine directly from PGA, independent of ribulose-1,5-bisphosphate oxidation, and that the route of serine synthesis via phosphohydroxypy ruvate and phosphoserine is the predominant pathway in these organisms .