Member of the two-component regulatory system PhoP/PhoQ that plays a role in the regulation of resistance towards polymyxin B and cationic antimicrobial peptides in response to limiting concentrations of Mg(2+) (PubMed:10564474, PubMed:11021929, PubMed:24994873). Functions as a transcriptional activator by direct binding to a cis-acting sequence upstream of the target gene promoters including oprH and pmrH promoters (PubMed:16707691). Deletion leads to loss of expression of the outer-membrane protein OprH which is part of the response towards Mg(2+) starvation.