Figure 5

Speculated routes of oxaloacetate and succinate replenishment in the thiotrophic B. azoricus symbiont. (a) Incomplete TCA cycle in thiotrophs and possible mechanism of oxaloacetate and succinate replenishment by host enzymes. Enzymes catalyzing reactions shown in grey are not encoded in the B. azoricus thiotroph genome. All host enzymes indicated here except AspC are highly abundant in the gill proteome of B. azoricus (*these host proteins were detected with significantly elevated expression levels in the symbiont-containing samples, as compared to the symbiont-free foot tissue, see also Table 1). For abundance of the thiotroph’s TCA cycle enzymes refer to Figure 4a. A citrate transporter, which might potentially import oxaloacetate (in exchange for an unknown substrate such as pyruvate or acetate indicated in the figure by an X), was not detected in the proteome but is encoded in the thiotroph’s genome. The depleted TCA intermediates oxaloacetate and succinate/succinyl-CoA are marked in bold. AspC: aspartate transaminase, Icl: isocitrate lyase, Mdh: malate dehydrogenase, Msh: malate synthase, ME: malic enzyme, OadA: oxaloacetate decarboxylase, Odh: 2-oxoglutarate dehydrogenase, PEP: phosphoenolpyruvate, PckA: PEP carboxykinase, Sdh: succinate dehydrogenase. Note that amino acid biosynthesis from oxaloacetate also involves succinyl-CoA, but that this interconnection of both pathways is not shown in the interest of clarity. (b) Overview of enzymes that can potentially transport or replenish oxaloacetate and succinate in different sulfur-oxidizing Gammaproteobacteria. Presence or absence of the corresponding enzyme gene (Gen) or protein (Prot) is indicated by + and −, respectively. *Data from this study. Bat: B. azoricus thiotroph, Bah: B. azoricus host (Bettencourt et al., 2010), CVo: Ca. Vesicomyosocius okutanii (Calyptogena okutanii symbiont, Kuwahara et al., 2007), CRm: Ca. Ruthia magnifica (C. magnifica symbiont, Newton et al., 2007), CEp: Ca. Endoriftia persephone (Riftia pachyptila symbiont, Markert et al., 2011), Sup: SUP05 (free-living chemoautotroph, Walsh et al., 2009), Arc: ARCTIC96BD-19 (Swan et al., 2011), Tc: Thiomicrospira crunogena Xcl-2 (Scott et al., 2006). (c) Some TCA cycle enzymes (dotted arrows 1–4) are missing in the B. azoricus thiotroph, in contrast to SUP05. See Supplementary Table S2B for genome assemblies included in the gene prediction for this figure.