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Translation initiation of the MgtL leader peptide is essential for the expression of mgtA in Escherichia coli

ABSTRACT: Riboswitches are cis-acting regulating RNA elements mostly found in the 5’-UTR of bacterial mRNAs. The Salmonella enterica mgtA riboswitch controls mgtA expression upon the sensing of intracellular Mg2+ ions. A leader peptide (MgtL) is contained within the mgtA riboswitch and its translation has been previously shown to be affected by Mg2+ concentrations. The current regulatory model suggests that the efficiency of MgtL translation is inversely related to mgtA transcription. Herein we show that the orthologue mgtA riboswitch of Escherichia coli also contains a leader peptide but, unlike S. enterica, that its translation is proportionally related to mgtA expression. Our results also suggest that ribosome stalling at mgtL is crucial for mgtA expression. Together, our results indicate that in E. coli, the expression of the MgtL leader peptide and mgtA are positively coordinated according to the intracellular Mg2+ concentration, a unique mechanism among known riboswitches.

ORGANISM(S): Escherichia coli K-12

PROVIDER: GSE203590 | GEO | 2023/05/01

REPOSITORIES: GEO

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Project description:In many Gram-negative bacteria, the stress sigma factor of RNA polymerase, σS/RpoS, remodels global gene expression to reshape the physiology of quiescent cells and ensure their survival under non-optimal growth conditions. In the foodborne pathogen Salmonella enterica serovar Typhimurium, σS is also required for biofilm formation and virulence. We have recently shown that a ΔrpoS mutation affects the magnesium content of Salmonella and expression of the primary Mg2+-transporter CorA. The other two Mg2+-transporters of Salmonella are encoded by the PhoP-activated mgtA and mgtB genes expressed under magnesium starvation, whereas the corA gene is expressed constitutively. The σS control of corA prompted us to evaluate the impact CorA in quiescent Salmonella cells, by using global and analytical proteomic analyses and physiological assays. The ΔcorA mutation conferred a competitive disadvantage to exit from stationary phase, and slightly impaired motility, but had no effect on total and free cellular magnesium contents. In contrast to the wild type strain, the ΔcorA mutant produced MgtA, but not MgtB, in the presence of high extracellular magnesium concentration. Under these conditions, MgtA production in the ΔcorA mutant did not require PhoP and, consistently, a ΔmgtA, but not a ΔphoP, mutation slightly reduced the magnesium content of the ΔcorA mutant. Nevertheless, the ΔphoP mutation strongly impaired growth and motility, in the absence of CorA only and independently of the extracellular magnesium concentration, unraveling the importance of PhoP-dependent functions in the absence of CorA. Under these conditions, several proteins involved in flagella formation, chemotaxis and secretion were affected by the ΔcorA and ΔphoP mutations in combination, but not alone. Altogether, our data pinpoint a regulatory network, where the absence of CorA is sensed and its physiological consequences alleviated by induction of MgtA- and PhoP- dependent compensatory mechanisms.

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Translation initiation of the MgtL leader peptide is essential for the expression of mgtA in Escherichia coli

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