Serine synthesis in cyanobacteria by a nonphotorespiratory pathway

Abstract

The pathway of serine synthesis was investigated in six species of cyanobacteria: Anabaena cylindrica, A. variabilis, A. flos‐aquae, Coccochloris peniocystis, Gleo‐capsa alpicola and Phormidium molle. Activity of the enzymes phosphoglycerate (PGA) dehydrogenase (EC 1.1.1.95), phosphoserine 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 could be detected. Metabolism of [1‐14C]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, respectively, Labelled serine isolated from these experiments was 100% carboxyl‐labelled indicating that it was formed directly from PGA. The labelling of serine was markedly reduced by 5 mM phosphoserine. These in vitro findings indicate that cyanobacteria are capable of synthesizing serine directly from PGA, independent of ribulose‐1.5‐bisphosphate oxidation, and that the route of serine synthesis via phosphohydroxypyruvate and phosphoserine is the predominant pathway in these organisms.