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Appl. Environ. Microbiol. doi:10.1128/AEM.00953-07
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Molecular characterization of the phaEC_Hm genes required for biosynthesis of poly(3-hydroxybutyrate) in the extremely halophilic archaeon Haloarcula marismortui

State Key Laboratory of Microbial Resources, Institute of Microbiology, Chinese Academy of Sciences, and Graduate University of Chinese Academy of Sciences, Beijing, P. R. China

^* To whom correspondence should be addressed. Email: xiangh{at}sun.im.ac.cn.

	Abstract

Although many haloarchaea produce biodegradable polyhydroxyalkanoates (PHAs), the genes involved in the PHA synthesis in the domain of Archaea were not experimentally investigated yet. In this study, we revealed that Haloarcula marismortui was able to accumulate poly(3-hydroxybutyrate) (PHB) up to 21% of cell dry weight (CDW) when cultured in a minimal medium with excessive glucose, and identified the phaE_Hm and phaC_Hm genes probably encoding two subunits of a Class III PHA synthase. These two genes were adjacent and directed by a single promoter located 26 bp upstream of the transcriptional start site, and were constitutively expressed under both nutrient-rich and -limited conditions. Interestingly, PhaC_Hm was revealed to be strongly bound with the PHB granules, but PhaE_Hm seemed not. Introduction of either phaE_Hm or phaC_Hm gene into Haloarcula hispanica, which harbored highly homologous phaEC_Hh genes, could enhance the PHB synthesis in the recombinant strains, while coexpression of the both genes always generated the highest PHB yield. Significantly, knockout of the phaEC_Hh genes in Har. hispanica led to a complete loss of the PHA synthase activity. Complementation with phaEC_Hm genes, but not a single one, restored the capability of PHB accumulation as well as the PHA synthase activity in this phaEC-deleted haloarchaeon. These results indicated that the phaEC genes were required for biosynthesis of PHB, and might encode an active PHA synthase in the Haloarcula species.

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