Elongation factor P restricts Salmonella\u2019s growth by controlling translation of a Mg2+ transporter gene during infection

Eunna Choi,Jung-Shin Lee,Daesil Nam,Soomin Choi,Shinae Park,Yoontak Han,Eun-Jin Lee

doi:10.1038/srep42098

Abstract

When a ribosome translates mRNA sequences, the ribosome often stalls at certain codons because it is hard to translate. Consecutive proline codons are such examples that induce ribosome stalling and elongation factor P (EF-P) is required for the stalled ribosome to continue translation at those consecutive proline codons. We found that EF-P is required for translation of the mgtB gene encoding a Mg2+ transporter in the mgtCBR virulence operon from the intracellular pathogen Salmonellaenterica serovar Typhimurium. Salmonella lacking EF-P decreases MgtB protein levels in a manner dependent on consecutive proline codons located in the mgtB coding region despite increasing transcription of the mgtCBR operon via the mgtP open reading frame in the leader RNA, resulting in an altered ratio between MgtC and MgtB proteins within the operon. Substitution of the consecutive proline codons to alanine codons eliminates EF-P-mediated control of the mgtB gene during infection and thus contributes to Salmonella’s survival inside macrophages where Salmonella experiences low levels of EF-P. This suggests that this pathogen utilizes a strategy to coordinate expression of virulence genes by an evolutionarily conserved translation factor.

Highlights

Mg2+ is involved in many important biological processes including coordinating nucleotides, stabilizing ribosome or membrane, and participating in many enzymatic reactions[1]
We report that Salmonella achieves altered MgtC and MgtB protein levels in the mgtCBR operon by another strategy and the altered MgtC and MgtB protein levels contribute to Salmonella virulence
When grown in media containing 500 μMMg2+ for 1 h to initiate transcription from the PhoP-dependent promoter, the efp mutant increased mRNA levels of both the mgtC and mgtB genes by ~100 fold (Fig. S1) and derepression in mRNA levels were not detected in a derivative strain with the mgtP proline codons substituted by threonine codons (Fig. S1)

Summary

Introduction

Mg2+ is involved in many important biological processes including coordinating nucleotides, stabilizing ribosome or membrane, and participating in many enzymatic reactions[1]. Salmonella lacking EF-P induces ribosome stalling at the consecutive proline codons in mgtP and allows the formation of stem-loop E structure, which enhances mgtCBR transcription (Fig. 1)[21]. This makes physiological sense because Salmonella decreases EF-P mRNA levels during infection[21] and it explains why the mgtC gene is highly expressed among other PhoP-regulated genes inside macrophages[22,23]. Salmonella lacking EF-P decreases MgtB protein levels via two consecutive proline codons located in the mgtB coding region despite increasing transcription of the entire mgtCBR operon by the leader RNA. Removal of EF-P-mediated control in the MgtB protein levels promotes Salmonella’s pathogenicity, implying that the ability to transport Mg2+ must be compromised during the course of infection

Methods

Results

Conclusion