Antisense transcription regulates the expression of the enterohemorrhagic Escherichia coli virulence regulatory gene ler in response to the intracellular iron concentration.

Toru Tobe,Naotake Ogasawara,Taku Oshima,Hiroki Takahashi,Yoko Kagayama,Hilo Yen,Martin G Marinus

doi:10.1371/journal.pone.0101582

Abstract

Enteric pathogens, such as enterohemorrhagic E. coli (EHEC) O157:H7, encounter varying concentrations of iron during their life cycle. In the gastrointestinal tract, the amount of available free iron is limited because of absorption by host factors. EHEC and other enteric pathogens have developed sophisticated iron-responsive systems to utilize limited iron resources, and these systems are primarily regulated by the Fur repressor protein. The iron concentration could be a signal that controls gene expression in the intestines. In this study, we explored the role of iron in LEE (locus for enterocyte effacement) virulence gene expression in EHEC. In contrast to the expression of Fur-regulated genes, the expression of LEE genes was greatly reduced in fur mutants irrespective of the iron concentration. The expression of the ler gene, the LEE-encoded master regulator, was affected at a post-transcription step by fur mutation. Further analysis showed that the loss of Fur affected the translation of the ler gene by increasing the intracellular concentration of free iron, and the transcription of the antisense strand was necessary for regulation. The results indicate that LEE gene expression is closely linked to the control of intracellular free iron homeostasis.

Highlights

Enteric pathogens that infect the mammalian gut use specific traits, referred to as virulence factors, to grow on the intestinal surface and sometimes pass through the epithelial barrier to reach deeper tissues
The expression of LEE genes was elevated in enterohemorrhagic E. coli (EHEC) grown in Dulbecco’s modified Eagle medium (DMEM), which contains only 0.25 mM Fe(NO3)3, compared with that in bacteria grown in LB medium, which contains,7.6 mM Fe [20]
We found that the virulence genes of EHEC are regulated by the intracellular iron concentration

Summary

Introduction

Enteric pathogens that infect the mammalian gut use specific traits, referred to as virulence factors, to grow on the intestinal surface and sometimes pass through the epithelial barrier to reach deeper tissues. To activate the expression of virulence factors at the appropriate time and to target niches, pathogens often sense the chemical and/or physical conditions of the intestinal environment. The ferric uptake regulator, is a repressor of genes involved in iron utilization [4]. In addition to iron utilizing systems, Fur and iron regulate a variety of genes involved in respiration, acid resistance, oxidative stress responses, and virulence [5]. The expression of virulence genes is known to be regulated by the iron concentration in a variety of enteric bacteria, such as Vibrio spp, Pseudomonas aeruginosa, Yersinia spp, Salmonella spp, and pathogenic Escherichia coli [6]. In enterohemorrhagic E. coli (EHEC), stx (Shiga-toxin) genes are directly regulated by Fur, and their expression is activated when the iron concentration decreases [7,8]

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