Studies on the influence of phasins on accumulation and degradation of PHB and nanostructure of PHB granules in Ralstonia eutropha H16

Phasins play an important role in the formation of poly(3-hydroxybutyrate) [PHB] granules and affect their size and number in the cells. Recent studies on the PHB granule proteome and analysis of the complete genomic DNA sequence of Ralstonia eutropha H16 have identified three homologues of the phasin protein PhaP1. In this study, mutants of R. eutropha deficient in the expression of the phasin genes phaP1, phaP2, phaP3, phaP4, phaP12, phaP123, and phaP1234 were examined by gas chromatography. In addition, the nanostructures of the PHB granules of the wild-type and of the mutants were imaged by atomic force microscopy (AFM), and the molecular masses of the accumulated PHB were analyzed by gel permeation chromatography. For this, cells were cultivated under conditions permissive for accumulation of PHB and were then cultivated under conditions permissive for degradation of PHB. Mutants deficient in the expression of phaP2, phaP3, or phaP4 genes mobilized the stored PHB only slowly like the wild-type, whereas degradation occurred much earlier and faster in the phaP1 single mutant as well as in all multiple mutants defective in the phaP1 gene plus one or more other phasin genes. This indicated that the presence of the major phasin PhaP1 on the granule surface is important for PHB degradation and that this phasin is therefore of particular relevance for PHB accumulation. It was also shown that the molecular weights of the accumulated PHB were identical in all examined strains; phasins have therefore no influence on the molecular weight of PHB. The AFM images obtained in this study showed that the PHB granules of R. eutropha H16 form a single interconnected system inside the wild-type cells.