Biotechnology journal | 2019
Microbial Secretion Platform for 3-Hydroxybutyrate Oligomers and Its End-Capped Forms Using Chain Transfer Reaction-Mediated Polyhydroxyalkanoate Synthases.
Abstract
The biodegradable polyester 3-hydroxybutyrate (3HB) polymer [P(3HB)] is intracellularly synthesized and accumulated in recombinant Escherichia coli harboring a set of polymer synthetic genes. In this study, we used native polyhydroxyalkanoate (PHA) synthases to attempt to microbially secrete 3HB homo-oligomers (3HBOs), which are widely distributed in nature as physiologically active substances such as plant growth-promoting factors and antibiotic compounds. High secretory production was observed, especially for the two PHA synthases from Aeromonas caviae and Bacillus cereus YB4. Surprisingly, an ethyl ester at the carboxy terminus (ethyl ester form) of 3HBOs was identified for most of the PHA synthases tested, as revealed by ESI-TOF-MS and NMR analyses. Next, 3HBOs with a functional carboxyl group (carboxyl form of 3HBO) were obtained by using the alcohol dehydrogenase gene (adhE)-deficient mutant strain, suggesting that the endogenous ethanol produced in E. coli acts as a chain transfer (CT) agent in the generation of 3HBOs. Furthermore, an in vitro polymerization assay revealed that CT agents such as ethanol and free 3HB are involved in the generation of ethyl ester form of 3HBO and carboxyl form of 3HBO, respectively. The microbial platform established herein allows the secretion of 3HBOs with desirable end structures by supplementation with various CT agents. The obtained 3HBOs and their end-capped forms may be used as physiologically active substances and building blocks for polymeric materials. This article is protected by copyright. All rights reserved.