Many enzymes require the molybdenum cofactor, Moco. Under Mo-limiting conditions, the high-affinity ABC transporter ModABC permits molybdate uptake and Moco biosynthesis in bacteria. Under Mo-replete conditions, Escherichia coli represses modABC transcription by the one-component regulator, ModE, consisting of a DNA-binding and a molybdate-sensing domain. Instead of a full-length ModE protein, many bacteria have a shorter ModE protein, ModE(S) , consisting of a DNA-binding domain only. Here, we asked how such proteins sense the intracellular molybdenum status. We show that the Agrobacterium tumefaciens ModE(S) protein Atu2564 is essential for modABC repression. ModE(S) binds two Mo-boxes in the modA promoter as shown by electrophoretic mobility shift assays. Northern analysis revealed cotranscription of modE(S) with the upstream gene, atu2565, which was dispensable for ModE(S) activity. To identify genes controlling ModE(S) function, we performed transposon mutagenesis. Tn5 insertions resulting in derepressed modA transcription mapped to the atu2565-modE(S) operon and several Moco biosynthesis genes. We conclude that A. tumefaciens ModE(S) activity responds to Moco availability rather than to molybdate concentration directly, as is the case for E. coli ModE. Similar results in Sinorhizobium meliloti suggest that Moco dependence is a common feature of ModE(S) regulators.
The hypoxia-inducible transcription factor 1 (HIF-1) has been shown to enhance microbial killing and to ameliorate the course of bacterial infections. While the impact of HIF-1 on inflammatory diseases of the gut has been studied intensively, its function in bacterial infections of the gastrointestinal tract remains largely elusive. With the help of a publicly available gene expression data set, we could infer significant activation of HIF-1 after oral infection of mice with
Salmonella
Typhimurium. Immunohistochemistry and western blot analysis confirmed marked HIF-1α protein stabilization, especially in the intestinal epithelium. This prompted us to analyze conditional
Hif1a
-deficient mice to examine cell type-specific functions of HIF-1 in this model. Our results demonstrate enhanced non-canonical induction of HIF-1 activity upon
Salmonella
infection in the intestinal epithelium as well as in macrophages. Surprisingly,
Hif1a
deletion in intestinal epithelial cells did not impact on inflammatory gene expression, bacterial spread or disease outcome. In contrast,
Hif1a
deletion in myeloid cells enhanced intestinal
Cxcl2
expression and reduced the cecal
Salmonella
load.
In vitro
, HIF-1α-deficient macrophages showed an overall impaired transcription of mRNA encoding pro-inflammatory factors, however, intracellular survival of
Salmonella
was not impacted by HIF-1α deficiency.
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