Reduction mechanism of polyhydroxyalkanoate (PHA) molecular weight by class IV PHA synthase

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Polyhydroxyalkanoate (PHA)-producing Bacillus strains possess class IV PHA synthase, which is composed of the subunits PhaR and PhaC. Recombinant Escherichia coli expressing PHA synthase from Bacillus cereus (PhaRC) showed an unusual reduction of the molecular weight of PHA produced during the stationary phase of growth. In this study, we revealed that the molecular weight reduction was the result of alcoholytic cleavage of PHA chains by PhaRC induced by endogenous ethanol. This scission reaction was also induced by exogenous ethanol in both in vivo and in vitro assays. By employing the non-ethanol producer as a host, high molecular weight PHA was successfully synthesized even when PhaRC was expressed.