This study investigates how mutations in loci other than kdpDE affect the regulation of the kdp operon in Escherichia coli. The kdp operon is responsible for potassium transport and is regulated by a dual-component phosphorelay system involving KdpD (a sensor kinase) and KdpE (a response regulator). The study identifies two mutations, dke-1 and dke-2, that significantly reduce kdp expression. dke-1 is located in the trxB gene, which encodes thioredoxin reductase, while dke-2 is in the hns gene, which encodes the nucleoid protein H-NS. These mutations affect the cytoplasmic thiol oxidation status and H-NS levels, which in turn influence the signal transduction pathway upstream of KdpD, thereby affecting the kinase activity of KdpD and the magnitude of kdp expression. The study also shows that mutations in trxB and hns do not affect the regulation of another dual-component system, EnvZ-OmpR, indicating that the effects of these mutations are specific to the kdp regulatory pathway. Additionally, the study demonstrates that the effects of trxB and hns mutations on kdp expression are not due to changes in growth rates, but rather to their effects on the signal transduction pathway. The results suggest that the cytoplasmic thiol oxidation status and H-NS levels play a critical role in the regulation of the kdp operon. The findings highlight the importance of these factors in the regulation of the kdp operon and suggest that they may be important in other regulatory processes as well. The study also shows that the effects of trxB and hns mutations on kdp expression are additive, indicating that these factors may act together to influence the regulation of the kdp operon. The study concludes that the regulation of the kdp operon is influenced by multiple factors, including the cytoplasmic thiol oxidation status and the nucleoid protein H-NS.This study investigates how mutations in loci other than kdpDE affect the regulation of the kdp operon in Escherichia coli. The kdp operon is responsible for potassium transport and is regulated by a dual-component phosphorelay system involving KdpD (a sensor kinase) and KdpE (a response regulator). The study identifies two mutations, dke-1 and dke-2, that significantly reduce kdp expression. dke-1 is located in the trxB gene, which encodes thioredoxin reductase, while dke-2 is in the hns gene, which encodes the nucleoid protein H-NS. These mutations affect the cytoplasmic thiol oxidation status and H-NS levels, which in turn influence the signal transduction pathway upstream of KdpD, thereby affecting the kinase activity of KdpD and the magnitude of kdp expression. The study also shows that mutations in trxB and hns do not affect the regulation of another dual-component system, EnvZ-OmpR, indicating that the effects of these mutations are specific to the kdp regulatory pathway. Additionally, the study demonstrates that the effects of trxB and hns mutations on kdp expression are not due to changes in growth rates, but rather to their effects on the signal transduction pathway. The results suggest that the cytoplasmic thiol oxidation status and H-NS levels play a critical role in the regulation of the kdp operon. The findings highlight the importance of these factors in the regulation of the kdp operon and suggest that they may be important in other regulatory processes as well. The study also shows that the effects of trxB and hns mutations on kdp expression are additive, indicating that these factors may act together to influence the regulation of the kdp operon. The study concludes that the regulation of the kdp operon is influenced by multiple factors, including the cytoplasmic thiol oxidation status and the nucleoid protein H-NS.