Schematic of the indirect uptake of folic acid by E. coli. Folic acid is composed of a central PABA moiety linked by a methylene bridge to a pteridine and to a single glutamic acid residue via a peptide bond. Folic acid is unstable and disassociates to generate PABA-glutamate (PABA-glu), which can further disassociate to PABA. E. coli is able to salvage PABA-glu via transport by the inner membrane protein, AbgT. PABA-glu is then cleaved intracellularly by the heterodimeric carboxypeptidase AbgA/B into PABA, where it can be used as a precursor for folate synthesis to generate the active form of folate, tetrahydrofolate (THF) which is used in the folate cycle. PABA can also diffuse across biological membranes. E. coli is also capable of de novo folate synthesis by generating PABA from chorismate and glutamine through the action of PabA, PabB and PabC. E. coli is unable to import folic acid directly.

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... bacteria can take up folates directly but many cannot. Instead they either make folate de novo or take up folate precursors such as para-amino benzoic acid (PABA, Figure 1, (Carter et al., 2007;LeBlanc et al., 2013;Magnusdottir et al., 2015). Folic acid and THFs are inherently unstable molecules, comprising of a central PABA moiety linked by a methylene bridge to a pteridine ring to form pteroic acid and by its carboxyl group to a L-glutamic acid residue by a peptide bond ( Green and Matthews, 2007). ...

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... doi: bioRxiv preprint Like several other Proteobacteria, E. coli is capable of de novo folate synthesis and the salvage of PABA and PABA-glu to make folate (Carter et al., 2007), but cannot take up intact folates (Nickerson and Webb, 1956;Webb, 1955) Figure 1). Here, we find that the major route of folic acid supplementation is an indirect route that this is dependent on the breakdown of folic acid into PABA-glu and subsequent uptake via the E. coli transporter, AbgT. ...

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... on C. elegans gcp-2.1 body length. Mutation of abgT alone did not influence growth of the gcp-2.1 (Supplementary Figure 1a). Rescue of gcp-2.1 developmental folate deficiency was achieved at a 10-fold lower concentration and independently of abgT expression, with the relatively stable THF, folinic acid (Supplementary Figure 2), suggesting that C. elegans takes up folinic acid directly. ...

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... with our previous findings (Virk et al., 2016), we find that C. elegans maintained on any E. coli pabA mutant are long-lived compared to C. elegans fed wild type E. coli (Figure 4, Supplementary Table 1), whereas the abgT mutation alone had no impact on C. elegans lifespan (P=0.4312, Supplementary Figure 1b). 10 µM folic acid was found to decrease C. elegans lifespan on pabA by 9.4% (P=0.0052), and decrease lifespan on the pabA mutant over-expressing abgT further (by 16.3%, P<0.0001, Figure 4a). ...

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... Figure 1. The E. coli abgT deletion has no effect on C. elegans development and lifespan a) body length of wild-type and gcp-2.1 mutant C. elegans at L4 stage raised on abgT mutant or wild-type E. coli (error bars represent standard deviation) b) survival curves of wild-type (glp-4) C. elegans on E. coli wild-type and abgT mutant. ...