Regulation of Escherichia coli Pathogenesis by Alternative Sigma Factor N

Abstract

σ^N (also σ⁵⁴) is an alternative sigma factor subunit of the RNA polymerase complex that regulates the expression of genes from many different ontological groups. It is broadly conserved in the Eubacteria with major roles in nitrogen metabolism, membrane biogenesis, and motility. σ^N is encoded as the first gene of a five-gene operon including rpoN (σ^N), ptsN, hpf, rapZ, and npr that has been genetically retained among species of Escherichia, Shigella, and Salmonella. In an increasing number of bacteria, σ^N has been implicated in the control of genes essential to pathogenic behavior, including those involved in adherence, secretion, immune subversion, biofilm formation, toxin production, and resistance to both antimicrobials and biological stressors. For most pathogens how this is achieved is unknown. In enterohemorrhagic Escherichia coli (EHEC) O157, Salmonella enterica, and Borrelia burgdorferi, regulation of virulence by σ^N requires another alternative sigma factor, σ^S, yet the model by which σ^N-σ^S virulence regulation is predicted to occur is varied in each of these pathogens. In this review, the importance of σ^N to bacterial pathogenesis is introduced, and common features of σ^N-dependent virulence regulation discussed. Emphasis is placed on the molecular mechanisms underlying σ^N virulence regulation in E. coli O157. This includes a review of the structure and function of regulatory pathways connecting σ^N to virulence expression, predicted input signals for pathway stimulation, and the role for cognate σ^N activators in initiation of gene systems determining pathogenic behavior.

Figure 1

Conservation and function of rpoN operon products. The rpoN operon is controlled by a single σ⁷⁰ promoter and consists of 5 genes (nucleotide length is given below each). See the text for a discussion of each product and its function. Percent nucleotide identity for each gene (entire ORF) is given for species of Escherichia, Shigella, and Salmonella relative to E. coli K-12 MG1655; the npr gene is absent in Salmonella.

Figure 2

Hypothesized interactions of σ^N and σ^S in B. burgdorferi, E. coli, and S. enterica. In B. burgdorferi, σ^N directly activates rpoS (encoding σ^S) transcription requiring the σ^N EBP Rrp2. In E. coli, σ^N indirectly reduces σ^S activity in a manner requiring σ^S antagonist FliZ, FlhDC, and σ^N EBP NtrC. In S. enterica, σ^S indirectly activates rpoN (encoding σ^N) transcription, requiring an unknown regulator(s). See the text for further details.

Figure 3

Structure of the σ^N-σ^S pathway regulating GDAR and LEE gene systems in E. coli O157. σ^N and cognate EBP NtrC initiate transcription of motility regulator flhD. FlhD activates transcription of σ^S antagonist, fliZ. FliZ repression of σ^S activity leads to downregulation of GDAR central regulator gadE and concomitant upregulation of the LEE. Increased LEE expression is predicted to occur through increased pchA or decreased gadE transcription. GadE activates transcription of GDAR system glutamate (Glu) decarboxylases (gadA/gadB) for catalytic acid detoxification, converting Glu to γ-aminobutyric acid (GABA).

Figure 4

Role for σ^N activators (EBPs) QseF and NtrC in the regulation of E. coli O157 pathogenesis. The σ^N EBPs QseF and NtrC activate σ^N-dependent virulence gene regulation in E. coli O157 in response to discrete input signals. The QseF-dependent pathway is sensitive to norepinephrine/epinephrine (NEPI/EPI) and autoinducer 3 (AI-3), whereas NtrC responds to nitrogen (N) and acetyl phosphate (AcP). Both QseF and NtrC positively regulate attaching and effacing (AE) lesion formation through different regulatory pathways (see text for details). QseF enhances AE formation by activation of espF_U (also known as tccP), whereas NtrC enhances AE formation by decreasing σ^S activity. Pathways may intersect at glnB. QseF can activate glnB transcription, the product of which (PII) interferes with phosphorylation of NtrC by sensor kinase NtrB. NtrC can also be activated by AcP. QseF is activated by sensor kinase QseE, as well as several noncognate sensor kinases. See the text for further details. P, Phosphate; LEE, locus of enterocyte effacement; GDAR, glutamate-dependent acid resistance.