Two-component regulatory systems can interact to process multiple environmental signals

Abstract

The PhoP/PhoQ two-component system of Salmonella typhimurium governs transcription of some 25 loci in response to the extracellular concentration of Mg 2؉. We have now identified one of these loci as pmrCAB, which codes for a two-component system that mediates resistance to the antibiotic polymyxin B. Transcription of seven of 25 PhoP-activated loci was dependent on a functional PmrA protein, the response regulator of the PmrA/PmrB system. Expression of the PmrA-dependent loci was induced by either Mg 2؉ limitation or mild acidification, whereas transcription of a PmrA-independent gene was activated by Mg 2؉ limitation but not acid pH. Induction of PmrA-activated genes by Mg 2؉ limitation required the PhoP and PhoQ proteins. In contrast, the acid-mediated activation of PmrA-regulated genes occurred in strains that were missing either one of these proteins. Transcriptional regulation by a cascade of two-component systems allows pathogenic bacteria to express their virulence determinants in response to a broader spectrum of environmental cues.

Molecular Characterization of the PmrA Regulon

“…2), consistent with previous results using lac gene fusions (9). That identical transcription start sites were detected when expression of PmrA-activated genes was induced by growth in low magnesium or by mild acidification suggests that these signals act by modifying the activity of the PmrA protein and argues against a model in which different promoters are used in response to different signals.…”

“…But transcription of PmrA-activated genes can also be induced by growth in mild acidic conditions in a PhoP/ PhoQ-independent manner mechanism (9 -11). To distinguish between PhoP/PhoQ-dependent and -independent transcription of PmrA-activated genes, total RNA was isolated from mid-exponential phase cultures of wild-type Salmonella grown in N-minimal media with 10 M Mg 2ϩ , pH 7.7, or 10 mM Mg 2ϩ , pH 5.8, respectively (9). RNA was also isolated from bacteria grown in N-minimal media, 10 mM Mg 2ϩ , pH 7.7, a condition that does not promote expression of PmrA-activated genes.…”

“…Transcription of PmrA-activated genes is induced in response to mild acidic conditions (9) or during growth in a low extracellular magnesium media in a process that requires the PhoP/PhoQ two-component system (10,11) (Fig. 1).…”

Closing the loop: The PmrA/PmrB two-component system negatively controls expression of its posttranscriptional activator PmrD

“…This makes sense because inhibiting the first step in the pathway (i.e., PhoQ activation) would compromise expression of the large number of genes that encompass the PhoP regulon (34,35). Furthermore, because the PmrA͞PmrB system can be activated independently of pmrD by mild acid pH or Fe 3ϩ (2,11,13), continuous PmrD production under such conditions might interfere with turning off expression of PmrAactivated genes when the environment changes to noninducing conditions. Autoregulatory feedback loops in which a protein negatively, or less often positively, controls its own transcription are common in bacteria (36).…”

“…Transcription of PmrA-activated genes is coordinately induced by extracytoplasmic Fe 3ϩ , which is sensed by the PmrB protein (2), and by extracytoplasmic low Mg 2ϩ (11), which is sensed by the PhoQ protein (ref. 12; Fig.…”