A signal transduction system that responds to extracellular iron

Iron is essential for all organisms but can be toxic in excess. Iron homeostasis is typically regulated by cytoplasmic iron binding proteins, but here we describe a signal transduction system (PmrA/PmrB) that responds to extracytoplasmic ferric iron. Iron promoted transcription of PmrA-activated genes and resistance to the antibiotic polymyxin in Salmonella. The PmrB protein bound iron via its periplasmic domain which harbors two copies of the sequence ExxE, a motif present in the Saccharomyces FTR1 iron transporter and in mammalian ferritin light chain. A pmrA mutant was hypersensitive to killing by iron but displayed wild-type resistance to a variety of oxidants, suggesting PmrA/PmrB controls a novel pathway mediating the avoidance of iron toxicity.

A small protein that mediates the activation of a two-component system by another two-component system

The PmrA-PmrB two-component system of Salmonella enterica controls resistance to the peptide antibiotic polymyxin B and to several antimicrobial proteins from human neutrophils. Transcription of PmrA-activated genes is induced by high iron, but can also be promoted by growth in low magnesium in a process that requires another two-component system, PhoP-PhoQ. Here, we define the genetic basis for the interaction between the PhoP-PhoQ and PmrA-PmrB systems. We have identified pmrD as a PhoP-activated gene that mediates the transcriptional activation of PmrA-regulated genes during growth in low magnesium. When transcription of pmrD is driven from a heterologous promoter, expression of PmrA-activated genes occurs even at repressing magnesium concentrations and becomes independent of the phoP and phoQ genes. The PmrD effect is specific for PmrA-regulated genes and requires functional PmrA and PmrB proteins. A pmrD mutant is sensitive to polymyxin if grown in low magnesium, but resistant if grown in high iron. The PmrD protein controls the activity of the PmrA-PmrB system at a post-transcriptional level.

Molecular characterization of the PmrA regulon

The two-component system PmrA/PmrB of Salmonella enterica controls expression of several loci including those mediating modifications in the lipopolysaccharide that result in polymyxin resistance. To gain insight in the regulation of polymyxin resistance, we mapped the transcription start sites of the PmrA-regulated genes pmrC, pmrG, pbgPE, and ugd and identified a conserved sequence in the promoter region of the first three genes. His-tagged PmrA protein could gel shift DNA fragments containing the promoters of the pmrC, pmrG, and pbgPE genes but not the udg promoter. DNase I footprinting analysis of the pmrC, pmrG, and pbgPE promoters indicate that phosphorylated as well as unphosphorylated PmrA bind to a 16-base pair imperfect inverted repeat sequence (5'-TTAAKTTCTTAAKGTT-3'), which is found 40, 80, and 38 nucleotides upstream from the transcription start sites of the pmrC, pmrG, and pbgPE genes, respectively. Our data suggest that a PmrA dimer activates transcription of the divergent pmrG and pbgPE promoters by binding to a single site in the pmrG-pbgPE intergenic region and that the ugd gene is regulated by the PmrA/PmrB system only indirectly.

Eubacterial sigma-factors

The initiation of transcription is the most important step for gene regulation in eubacteria. To initiate transcription, RNA polymerase has to associate with a small protein, known as a sigma-factor. The sigma-factor directs RNA polymerase to a specific class of promoter sequences. Most bacterial species synthesize several different sigma-factors that recognize different consensus sequences. This variety in sigma-factors provides bacteria with the opportunity to maintain basal gene expression as well as for regulation of gene expression in response to altered environmental or developmental signals. This review focuses on the function, regulation and distribution of the 14 different classes of sigma-factors that are presently known.

Eubacterial sigma-factors

The initiation of transcription is the most important step for gene regulation in eubacteria. To initiate transcription, RNA polymerase has to associate with a small protein, known as a sigma-factor. The sigma-factor directs RNA polymerase to a specific class of promoter sequences. Most bacterial species synthesize several different sigma-factors that recognize different consensus sequences. This variety in sigma-factors provides bacteria with the opportunity to maintain basal gene expression as well as for regulation of gene expression in response to altered environmental or developmental signals. This review focuses on the function, regulation and distribution of the 14 different classes of sigma-factors that are presently known.

Cloning and characterization of the gene encoding the primary sigma-factor of Campylobacter jejuni

The rpoD gene encoding the primary sigma-factor of Campylobacter jejuni was amplified from genomic DNA with degenerate oligonucleotide primers. The complete gene encodes a polypeptide of 622 amino acids and has a deduced M(r) of 72.6 kDa. This polypeptide is 40% identical to the RpoD (sigma 70) protein of Escherichia coli and has 66% identity with the Helicobacter pylori RpoD protein. A C. jejuni sigma 70 promoter, not recognized by the E. coli sigma 70 factor, could be activated in this bacterium in the presence of the cloned C. jejuni RpoD protein.

Identification of Campylobacter jejuni promoter sequences

A promoterless lacZ shuttle vector, which allowed screening of promoters by beta-galactosidase activity in Campylobacter jejuni and Escherichia coli, was developed. Chromosomal DNA fragments from C. jejuni were cloned into this vector; 125 of 1,824 clones displayed promoter activity in C. jejuni. Eleven clones with strong promoter activity in C. jejuni were further characterized. Their nucleotide sequences were determined, and the transcriptional start sites of the putative promoters in C. jejuni were determined by primer extension. Only 6 of these 11 promoters were functional in E. coli. The 11 newly characterized and 10 previously characterized C. jejuni promoters were used to establish a consensus sequence for C. jejuni promoters. The 21 promoters were found to be very similar. They contain three conserved regions, located approximately 10, 16, and 35 bp upstream of the transcriptional start point. The -10 region resembles that of a typical sigma70 E. coli promoter, but the -35 region is completely different. In addition a -16 region typical for gram-positive bacteria was identified.

Cloning and characterization of the lytB gene of Campylobacter jejuni

LytB of Escherichia coli is an essential gene involved in penicillin tolerance and the stringent response. The lytB gene of Campylobacter jejuni was cloned and characterized. It could complement a temperature-sensitive E. coli lytB mutant. The C. jejuni lytB gene encodes a protein of 277 amino acids that has 34, 36 and 40% amino acid identity with the LytB proteins of E. coli, Haemophilus influenzae, and Synechocystis sp. PCC6803, respectively. The lytB gene is situated between the aroA gene and a gene that encodes ribosomal protein S1.

The aroA gene of Campylobacter jejuni

The gene for 5-enolpyruvylshikimate-3-phosphate (EPSP) synthase (aroA) cloned from Campylobacter jejuni (Cj) strain 81116 was identified by complementation of an Escherichia coli (Ec) auxotrophic aroA mutant. The Cj aroA gene has been sequenced. It encodes an enzyme of 428 amino acids (aa), that is homologous to other bacterial EPSP synthases, especially that of Bacillus subtilis with which it has a 39% aa identity. The transcriptional start point was mapped. It is present in an upstream open reading frame (ORF) that has a strong homology to the gene encoding phenylalanine tRNA synthetase (pheS). Downstream from aroA another ORF is present which is homologous to the lytB gene of Ec. The stop codon of the aroA gene overlaps the start codon of lytB.