Abstract
Gene regulation in bacteria is primarily controlled at the level of transcription initiation by modifying the affinity of the RNA polymerase (RNAP) for the promoter. This control often occurs through the substitution of the RNAP sigma (σ) subunit. Next to the primary σ factor, most bacteria contain a variable number of alternative σ factors of which the extracytoplasmic function group (σECF) is predominant. Pseudomonas aeruginosa contains nineteen σECF, including the virulence regulator σVreI. σVreI is encoded by the vreAIR operon, which also encodes a receptor-like protein (VreA) and an anti-σ factor (VreR). These three proteins form a signal transduction pathway known as PUMA3, which controls expression of P. aeruginosa virulence functions. Expression of the vreAIR operon occurs under inorganic phosphate (Pi) limitation and requires the PhoB transcription factor. Intriguingly, the genes of the σVreI regulon are also expressed in low Pi despite the fact that the σVreI repressor, the anti-σ factor VreR, is also produced in this condition. Here we show that although σVreI is partially active under Pi starvation, maximal transcription of the σVreI regulon genes requires the removal of VreR. This strongly suggests that an extra signal, probably host-derived, is required in vivo for full σVreI activation. Furthermore, we demonstrate that the activity of σVreI is modulated not only by VreR but also by the transcription factor PhoB. Presence of this regulator is an absolute requirement for σVreI to complex the DNA and initiate transcription of the PUMA3 regulon. The potential DNA binding sites of these two proteins, which include a pho box and −10 and −35 elements, are proposed.
Highlights
Regulation of gene expression allows bacteria to adapt rapidly to alterations in their environment
The PUMA3-regulated promoters PpdtA and PphdA were active in low inorganic phosphate (Pi) and both the PhoB and σVreI proteins were required for such activation (Figure 1B)
Expression from PpdtA and PphdA correlates with σVreI production, which occurs in the wild-type PAO1 and vreA strains upon growth in low Pi but not in high Pi and does not occur in the phoB mutant (Figure 1C)
Summary
Regulation of gene expression allows bacteria to adapt rapidly to alterations in their environment. This regulation occurs primarily at the level of transcription initiation by modifying promoter recognition of the RNA polymerase (RNAP) holoenzyme. All bacteria contain a primary σ factor (i.e., σ70) that recognizes similar target promoter sequences and controls expression of genes required for general functions. Most bacteria contain several alternative σ factors that recognize alternative promoter sequences and activate expression of functions required only under specific circumstances (Ishihama, 2000). The promoter recognition of the RNAP is modulated first by substitution of the σ subunit and secondly by the interaction with transcription factors
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