CONTROL OF POLY-BETA-HYDROXYBUTYRATE SYNTHASE MEDIATED BY ACETYL PHOSPHATE IN CYANOBACTERIA

Poly-beta-hydroxybutyrate (PHB) synthesis in a cyanobacterium, Synecho coccus sp. strain MA19, is controlled at the enzyme level and is depen dent on the C/N balance in the culture medium, The control involves at least two enzymes, The first enzyme is PHB synthase. Little PHB synth ase activity,vas detected in crude extracts from cells grown under nit rogen-sufficient conditions (MA19(+N)). The activity was detected excl usively in membrane fractions from nitrogen-deprived cells (MA19(-N)) under light but not dark conditions. The shift in the enzyme activity was insensitive to chloramphenicol, which suggests posttranslational a ctivation, Acetyl phosphate activated PHB synthase in membrane fractio ns from MA19(+N), In vitro, the activation level of PHB synthase chang ed, depending on the concentration of acetyl phosphate, The second enz yme was phosphotransacetylase (EC 2.3.1.8), which catalyzes the conver sion of acetyl coenzyme A (acetyl-CoA) to acetyl phosphate, The activi ty was detected in elude extracts from MA19(-N) but not in those from MA19(+N). The results suggested that intracellular acetyl phosphate co ncentration could be controlled, depending on CIN balance and intracel lular acetyl-CoA concentration. Acetyl phosphate probably acts as a si gnal of C/N balance affecting PHB metabolism in MA19.