D. Facciotti, R. Valentine, J. Metz, K. Lardizabal, W. Schreckengost, V. Knauf, M. Lassner, C. Leveridge, A. Yamada, K. Yazawa
In the past ten years there has been an increasing number of reports on the presence in marine bacteria of long chain polyunsaturated fatty acides (PUFA): arachidonic acid (ARA= C20:4), eicosapentanoic acid (EPA=C20:5) and docosahexanoic acid (DHA=C22:6), which are usually absent in procaryotes. The potential for nutritional applications of these fatty acids has promoted research interest in these marine bacteria either for PUFA production or as source of PUFA genes. Recently a cluster of genes isolated from an EPA producing Shewanella putrefaciens (SCRC-2738) directed the synthesis of this fatty acid after its transfer into E. coli (K Yazawa: Lipids, Vol. 31 Supplement, S-297-300, 1996). At Calgene/Monsanto we have extended the study of PUFA synthesis to other bacteria such as Vibrio marinus which is a DHA producer, as well as two EPA producers: a Photobacter (SS9) and a Shewanella (SC2A). We will present the results of these studies summarized as follows: A). Marine bacteria from different genera share a unique gene system for PUFA synthesis. This system includes five genes, four of which are clustered as a possible operon. Strong homologies in both DNA composition and organization are conserved among genera. B). This PUFA system differs considerably from that of eucaryotes in that it is not oxygen dependent and can function under anaerobic conditions. C). The PUFA operon from Vibrio marinus was fully sequenced and transferred into E. coli where it made possible the synthesis of DHA. D). PUFA genes from either DHA or EPA producing bacteria can replace and / or complement each other in E. coli. A single gene (orf 8) determines the synthesis of either EPA or DHA according to the EPA or DHA phenotype of its bacterial source. E). In addition to studying the PUFA gene system we also started to investigate the physiological role played by PUFA in procaryotes. Chemically induced shewanella (SCRC-2738) mutants impaired in EPA synthesis were unable to grow at low temperatures (2 C) unless supplemented with EPA or other polyunsaturated fatty acids. Low temperature growing revertants, however, were frequently isolated from these mutants, the revertants did not synthesize PUFA but displayed higher levels of palmitoleic (C16:1) and vaccenic (C18:1) acids. This suggested that PUFA may be only an alternative to monounsaturated fatty acids for adaptation to low temperatures, unless their role is essential to non-vital functions that have yet to be identified.
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