HmuP Is a Coactivator of Irr-Dependent Expression of Heme Utilization Genes in Bradyrhizobium japonicum

• Published in: Journal of Bacteriology Vol. 194; no. 12; pp. 3137 - 3143
• Main Authors: Escamilla-Hernandez, Rosalba, O'Brian, Mark R
• Format: Journal Article
• Language: English
• Published: Washington, DC American Society for Microbiology 01-06-2012
• Subjects: Bacterial Proteins - metabolism; Bacteriology; Biological and medical sciences; Bradyrhizobium - genetics; Bradyrhizobium - growth & development; Bradyrhizobium - metabolism; Bradyrhizobium japonicum; crosslinking; ferric chloride; Fundamental and applied biological sciences. Psychology; Gene Deletion; Gene expression; Gene Expression Regulation, Bacterial; Genetics; Gram-negative bacteria; heme; Heme - metabolism; heme iron; Immunoprecipitation; Iron; Iron - metabolism; loci; messenger RNA; Microbiology; Miscellaneous; mutants; Mutation; precipitin tests; Promoter Regions, Genetic; Protein Binding; regulon; RNA-protein interactions; Trans-Activators - genetics; Trans-Activators - metabolism; transcription (genetics); Transcription Factors - metabolism; Bacteria; Rhizobiaceae; Gene expression; Symbiont; Bradyrhizobium japonicum; Heme
• ISSN: 0021-9193; 1098-5530; 1067-8832
• DOI: 10.1128/JB.00071-12

Utilization of heme as an iron source by Bradyrhizobium japonicum involves induction of the outer membrane heme receptor gene hmuR and other genes within the heme utilization locus. Here, we discovered the hmuP gene located upstream of hmuR and transcribed divergently from it along with hmuTUV. hmuP encodes a small protein that accumulated under iron limitation and is transcriptionally controlled by the global iron-responsive regulator Irr, as were all genes within the heme utilization locus. Cross-linking and immunoprecipitation experiments showed that Irr occupies the hmuR-hmuP promoter in vivo. An hmuP mutant did not grow on heme as an iron source, but retained the ability to use ferric chloride. Correspondingly, induction of hmuR mRNA under iron limitation was severely diminished in an hmuP strain, but other genes within the Irr regulon were unaffected. HmuP occupied the hmuR-hmuP promoter, and thus it plays a direct regulatory role in gene expression. HmuP was not required for Irr occupancy, nor was ectopic expression of hmuP from an Irr-independent promoter sufficient to induce the hmuR gene. Thus, both HmuP and Irr occupancy are necessary for hmuR induction. We suggest that HmuP is a coactivator of Irr-dependent expression of hmuR.