The biosynthesis and incorporation of polyunsaturated fatty acids into phospholipid membranes is a unique feature of certain marine Gammaproteobacteria inhabiting high-pressure and/or low temperature environments. In these bacteria, monounsaturated and saturated fatty acids are produced via the classical dissociated Type II fatty acid synthase mechanism, while omega-3 polyunsaturated fatty acids such as EPA (20:5n-3) and DHA (22:6n-3) are produced by a hybrid polyketide/fatty acid synthase – encoded by the pfa genes - also referred to as the secondary lipid synthase mechanism. In this work, phenotypes associated with partial or complete loss of monounsaturated biosynthesis are shown to be compensated for by several-fold increased production of polyunsaturated fatty acids in the model marine bacterium Photobacterium profundum SS9. One route to suppression of these phenotypes could be achieved by transposition of insertion sequences within or upstream of the fabD, malonyl CoA-acyl carrier protein transacylase, coding sequence. Genetic experiments in this strain indicated that fabD is not an essential gene, yet mutations in fabD and pfaA are synthetically lethal. Based on these results, we speculated that the malonyl-CoA transacylase domain within PfaA compensates for loss of FabD activity. Heterologous expression of either pfaABCD from P. profundum SS9 or pfaABCDE from Shewanella pealeana in Escherichia coli complemented the loss of the chromosomal copy of fabD in vivo. The co-occurrence of independent, yet compensatory fatty acid biosynthetic pathways in select marine bacteria may provide genetic redundancy to optimize fitness under extreme conditions.
Importance
A defining trait among many cultured piezophilic and/or psychrophilic marine Gammaproteobacteria is the incorporation of both monounsaturated and polyunsaturated fatty acids into membrane phospholipids. The biosynthesis of these different classes of fatty acid molecules is linked to two genetically distinct co-occurring pathways that utilize the same pool of intracellular precursors. Using a genetic approach, new insights have been gained into the interactions between these two biosynthetic pathways. Specifically, core fatty acid biosynthesis genes previously thought to be essential were found to be non-essential in strains harboring both pathways due to functional overlap between the two pathways. These results provide new routes to genetically optimize long-chain omega-3 polyunsaturated fatty acid biosynthesis in bacteria and reveal a possible ecological role for maintaining multiple pathways for lipid synthesis in a single bacterium.
Fatty acid synthase: a novel target for antiglioma therapy
