Abstract
It is established that Escherichia coli β-ketoacyl-ACP synthase (KAS) I (encoded by EcfabB) is the primary, if not exclusive, factor for elongation of the cis-3-decenoyl-ACP (C10:1-ACP) but not effective with C16:1- or longer-chain-ACPs. To test the extent to which these features apply to KAS I proteins in other species, in this study, we examined the physiological role of FabB in Shewanella oneidensis, an excellent model for researching type II fatty acid synthetic (FAS) system and its regulation. We showed that the loss of either FabA (the enzyme that introduces double bond) or FabB, in the absence of DesA which desaturizes C16 and C18 to generate respective C16:1 and C18:1, leads to a UFA auxotroph. However, fatty acid profiles of membrane phospholipid of the fabA and fabB mutants are significantly different, suggesting that FabB participates in steps beyond elongation of C10:1-ACP. Further analyses demonstrated that S. oneidensis FabB differs from EcFabB in that (i) it is not the only enzyme capable of catalyzing elongation of the cis-3-decenoyl-ACP produced by FabA, (ii) it plays a critical role in elongation of C16:1- and longer-chain-ACPs, and (iii) its overproduction is detrimental.
Highlights
Bacterial membrane fluidity is largely determined by the relative levels of straight-chain saturated, unsaturated fatty acids (SFAs and UFAs), and branched-chain fatty acids (BCFAs) within the phospholipids of their membrane bilayers (Campbell and Cronan, 2001; de Mendoza, 2014)
The role is likely fulfilled by FabV, another type of enoyl-acyl carrier protein reductase (ENR) found in Vibrio cholerae and P. aeruginosa (Massengo-Tiassé and Cronan, 2008; Zhu et al, 2010)
Such a scenario is rather common in bacteria; for instance, B. subtilis possesses two functional FabH homologs and P. aeruginosa uses a new class of ketoacyl-ACP synthase (KAS) to initiate the fatty acid synthesis it has multiple FabH orthologs (Choi et al, 2000; Yuan et al, 2012)
Summary
Bacterial membrane fluidity is largely determined by the relative levels of straight-chain saturated, unsaturated fatty acids (SFAs and UFAs), and branched-chain fatty acids (BCFAs) within the phospholipids of their membrane bilayers (Campbell and Cronan, 2001; de Mendoza, 2014). Type II FAS pathway, the current knowledge of which derives mainly from model organism Escherichia coli (Cronan and Thomas, 2009), is initiated with an acetyl coenzyme A (acetyl-CoA) starter unit and a malonyl-acyl carrier protein (malonyl-ACP) extender unit (Figure 1). While acetylCoA is an intermediate of various metabolic pathways, malonyl-ACP is produced from acetyl-CoA through two sequential steps, catalyzed by acetyl-CoA carboxylase (ACC) and malonylCoA-ACP transacylase encoded by accABCD and fabD respectively (Li and Cronan, 1992; Serre et al, 1995). The condensation of malonyl-ACP with the growing acyl chain is catalyzed by KAS I (encoded by fabB) and KAS II (encoded by fabF; (Garwin et al, 1980a; Ulrich et al, 1983; Heath and Rock, 2002)
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