| Summary: | Expression of the Pseudomonas aeruginosa outer membrane protein OprH is
induced in low magnesium growth conditions (Nicas and Hancock, 1980; 1983). This
protein has been proposed to play a role in stabilizing the outer membrane in the absence
of Mg²⁺ by interacting with LPS at sites where these cations would bind. Adaptation to
magnesium limitation in Salmonella typhimurium has been shown to occur through
activation of the two-component regulatory system, PhoP-PhoQ (Soncini et al., 1996).
Putative PhoP and PhoQ proteins were identified in the P. aeruginosa genome through
homology searches using the corresponding S. typhimurium protein sequences. The genes
encoding these proteins were located directly downstream of the gene encoding OprH.
Transcriptional linkage of oprH, phoP and phoQ was demonstrated and the hypothesis
that this system regulates expression of OprH in P. aeruginosa was tested in the
following study.
Through construction of aphoP null mutants and transformation of this mutant
with PhoP encoding plasmids, it was shown that PhoP is required for expression of OprH.
Furthermore, PhoP was demonstrated to be an activator of oprH, phoP and phoQ
transcription from a promoter upstream of oprH. In contrast, a phoQ null mutant showed
high-level, unregulated activation of oprH and phoP transcription and OprH expression.
Complementation of this mutant demonstrated a requirement for PhoQ in down
regulation of transcription and response to magnesium. Analysis of the oprH promoter
enabled identification of the start of transcription and delineation of the sequences
required for regulated OprH expression to within 90 basepairs of the ATG.
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