The iron-sensing aconitase B binds its own mRNA to prevent sRNA-induced mRNA cleavage.
Bottom Line: In Escherichia coli, aconitase B (AcnB) is a typical moonlighting protein that can switch to its apo form (apo-AcnB) which favors binding its own mRNA 3'UTR and stabilize it when intracellular iron become scarce.Whereas RyhB can block acnB translation initiation, RNase E-dependent degradation of acnB was prevented by apo-AcnB binding close to the cleavage site.This previously uncharacterized regulation suggests an intricate post-transcriptional mechanism that represses protein expression while insuring mRNA stability.
Affiliation: Department of Biochemistry, RNA Group, University of Sherbrooke, 3201 Jean Mignault Street, Sherbrooke, Quebec J1E 4K8, Canada.Show MeSH
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Mentions: A common outcome of sRNAs binding to their target mRNAs is rapid RNase E-mediated cleavage of the targets. However, the RyhB-induced acnB mRNA degradation that has been previously reported (15,31) can be prevented when cells are grown under low-iron conditions. Combined with previous studies, data presented here suggested that reduced intracellular iron promoted the switch of holo-AcnB to apo-AcnB with the resulting RNA-binding of acnB mRNA 3′UTR and protection against RyhB-induced mRNA degradation. According to the model proposed here, RyhB still binds to acnB RBS and partially blocks its translation initiation under low-iron conditions. However, our data support the interpretation that bound apo-AcnB seems to interfere with RNase E access to acnB mRNA cleavage site (Figure 8). Accordingly, this mechanism of regulation allows long-term expression homeostasis of AcnB as opposed to a significant stronger expression of AcnB in the absence of RyhB sRNA (Figure 6A). Although RyhB partially blocked acnB translation (Figures 4B and 6A, right), there was a steady production of AcnB (Figure 6A, left). This seemingly paradoxical observation may be explained by the functional nature of AcnB. When cells grow under iron starvation, most RyhB targets become degraded. These degraded targets encode single-function enzymes, whose activity would falter in the absence of sufficient amounts of iron as cofactor. In contrast, bifunctional AcnB switches functions despite low intracellular iron level. When it adopts its catalytic conformation, aconitase has an iron-dependent metabolic function that participates to the TCA cycle. As an RNA-binding protein, apo-AcnB regulates expression of various transcripts such as acnB and ftsH (3) despite environmental variations of iron concentrations and maintains steady expression of acnB that allows sufficient quantities of cellular AcnB to modulate both activities.
Affiliation: Department of Biochemistry, RNA Group, University of Sherbrooke, 3201 Jean Mignault Street, Sherbrooke, Quebec J1E 4K8, Canada.