Carbon dioxide fixation in Helminthosporium cynodontis

A mycelial suspension of Helminthosporium cynodontis (ATCC24938), grown on glucose-peptone-yeast extract broth and exposed to NaH14CO3 for 5 h, fixed significant quantities of 14C into the following fractions (%): small molecular weight components, 7-4; lipid and lipoproteins, 3-9; nucleic acids, 59; the residual protein and cell wall fragments, 29-2. The labelled protein components were (%): aspartate, 39; glutamate, 18; cystine, 15; threonine, 9. Radioactive nucleic acid components were (%): adenine, 18; guanine, 18; cytidylate, 34; uridylate, 30. When the mycelium was grown in Czapek-Dox glucose medium and incubated in this medium plus NaH14CO3, the nucleic acid fraction contained 29-9% and the residual protein 49-5% of the cellular radioactivity. The removal of CO2 from the atmosphere did not reduce growth. Pyruvate carboxylase (PC) and phosphoenolypyruvate carboxykinase (PEPCK) activities were demonstrated in extracts of H. cynodontis. Synthesis of PEPCK was stimulated under conditions promoting gluconeogenesis and was reduced under conditions promoting glycolysis, while PC synthesis was similar under both conditions.