Photosynthetic accumulation of poly-(hydroxybutyrate) by cyanobacteria - the metabolism and potential for CO2 recycling

Regulatory mechanism in PHB [poly-(hydroxybutyrate)] accumulation by cyanob acteria, especially by a thermophilic isolate, Synechococcus MA19 was revie wed in comparison with a genetically engineered strain. The strain, MA19 ac cumulates PHB under nitrogen starved and photoautotrophic conditions (MA19- N). Little PHB synthase activity was detected in crude extracts from the ce lls grown in nitrogen sufficient conditions (MA19 + N). The activity was de tected exclusively in membrane fractions from MA19 + N. The change of the e nzyme activity was insensitive to chloramphenicol, which suggests post-tran slational activation. In vitro, acetyl phosphate activated PHB synthase in membrane fractions from MA19 + N, and the extent of activation depended on the concentration of acetyl phosphate. Phosphotransacetylase which catalyze s the conversion of acetyl-CoA to acetyl phosphate was detected in crude ex tracts from MA19-N but not in those from MA19 + N. These results suggested that intracellular acetyl phosphate concentration could be controlled, depe nding on C-N balance and intracellular acetyl-CoA concentration. On the con trary, in genetically-engineered cyanobacterium (transformant with PHB synt hesizing genes from Ralstonia eutropha), it did not seem to be PHB synthase but acetyl-CoA flux that limits PHB synthesis. The closer association of P HB granules with thylakoid membranes in MA19 is suggested than that in the genetically-engineered cyanobacterium, which may reflect the difference of distribution of PHB synthase. Transposon-mutagenesis was used to acquire mu tants of its altered PHB regulatory mechanism. PHA production by cyanobacte ria was considered from the aspects of photobioreactors. (C) 1999 Elsevier Science B.V. All rights reserved.