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Structure and regulation of the omega-3 polyunsaturated fatty acid synthase genes from the deep-sea bacterium Photobacterium profundum strain SS9

Center for Marine Biotechnology and Biomedicine, Marine Biology Research Division, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093-0202, USA¹

Author for correspondence: Douglas H. Bartlett. Tel: +1 858 534 5233. Fax: +1 858 534 7313. e-mail: dbartlett{at}ucsd.edu

Omega-3 polyunsaturated fatty acids (PUFAs) such as eicosapentaenoic acid (20:5n-3; EPA) and docosahexaenoic acid (22:6n-3; DHA) have been shown to be of major importance in the promotion of cardiovascular health, proper human development and the prevention of some cancers. A high proportion of bacterial isolates from low-temperature and high-pressure marine environments produce EPA or DHA. This paper presents the sequence of a 33 kbp locus from the deep-sea bacterium Photobacterium profundum strain SS9 which includes four of the five genes required for EPA biosynthesis. As with other bacterial pfa (polyunsaturated fatty acid) genes, the deduced amino acid sequences encoded by the SS9 genes reveal large multidomain proteins that are likely to catalyse EPA biosynthesis by a novel polyketide synthesis mechanism. RNase protection experiments separated the SS9 pfa genes into two transcriptional units, pfaA–C and pfaD. The pfaA transcriptional start site was identified. Cultivation at elevated hydrostatic pressure or reduced temperature did not increase pfa gene expression despite the resulting increase in percentage composition of EPA under these conditions. However, a regulatory mutant was characterized which showed both increased expression of pfaA–D and elevated EPA percentage composition. This result suggests that a regulatory factor exists which coordinates pfaA–D transcription. Additional consideration regarding the activities required for PUFA synthesis is provided together with comparative analyses of bacterial pfa genes and gene products.

Keywords: eicosapentaenoic acid, pfa genes, hydrostatic pressure, polyketide synthase, ribonuclease protection assay

Abbreviations: ACP, acyl carrier protein; AT, acyl CoA:ACP transacylase; CLF, chain length factor; DHA, docosahexaenoic acid; DH/I, dehydratase/isomerase; EPA, eicosapentaenoic acid; ER, enoyl reductase; FAS, fatty acid synthase; KR, ß-ketoacyl-ACP reductase; KS, ß-ketoacyl-ACP synthase; PKS, polyketide synthase; PPTase, phosphopantetheinyl transferase; PUFA, polyunsaturated fatty acid; RPA, ribonuclease protection assay

The GenBank accession numbers for the sequences reported in this paper are AF409100 and AF467805.

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