title: Characterization of the KstR-dependent promoter of the gene for the first step of the cholesterol degradative pathway in Mycobacterium smegmatis.
abstract: The KstR-dependent promoter of the MSMEG_5228 gene of Mycobacterium smegmatis, which encodes the 3-β-hydroxysteroid dehydrogenase (3-β HSD(MS)) responsible for the first step in the cholesterol degradative pathway, has been characterized. Primer extension analysis of the P₅₂₂₈ promoter showed that the transcription starts at the ATG codon, thus generating a leaderless mRNA lacking a 5' untranslated region (5'UTR). Footprint analyses demonstrated experimentally that KstR specifically binds to an operator region of 31 nt containing the quasi-palindromic sequence AACTGGAACGTGTTTCAGTT, located between the -5 and -35 positions with respect to the transcription start site. This region overlaps with the -10 and -35 boxes of the P₅₂₂₈ promoter, suggesting that KstR represses MSMEG_5228 transcription by preventing the binding of RNA polymerase. Using a P₅₂₂₈-β-galactosidase fusion we have demonstrated that KstR is able to work as a repressor in a heterologous system like Escherichia coli. A 3D model of the KstR protein revealed folding typical of TetR-type regulators, with two domains, i.e. a DNA-binding N-terminal domain and a regulator-binding C-terminal domain composed of six helices with a long tunnel-shaped hydrophobic pocket that might interact with a putative highly hydrophobic inducer. The finding that similar P₅₂₂₈ promoter regions have been found in all mycobacterial strains examined, with the sole exception of Mycobacterium tuberculosis, provides new clues about the role of cholesterol in the pathogenicity of this micro-organism.
authors.affiliation: Department of Environmental Biology, Centro de Investigaciones Biológicas, Consejo Superior de Investigaciones Científicas, Madrid, Spain.
authors.forename: Iria
authors.initials: I
authors.lastname: Uhía
authors.affiliation: Department of Environmental Biology, Centro de Investigaciones Biológicas, Consejo Superior de Investigaciones Científicas, Madrid, Spain.
authors.forename: Beatriz
authors.initials: B
authors.lastname: Galán
authors.affiliation: Department of Chemical and Physical Biology, Centro de Investigaciones Biológicas, Consejo Superior de Investigaciones Científicas, Madrid, Spain.
authors.forename: Francisco Javier
authors.initials: FJ
authors.lastname: Medrano
authors.affiliation: Department of Environmental Biology, Centro de Investigaciones Biológicas, Consejo Superior de Investigaciones Científicas, Madrid, Spain.
authors.forename: José Luis
authors.initials: JL
authors.lastname: García
mesh_terms: D015096:3-Hydroxysteroid Dehydrogenases / Q000235:genetics* / Q000378:metabolism; D001483:Base Sequence; D002784:Cholesterol / Q000378:metabolism*; D015964:Gene Expression Regulation, Bacterial; D053858:Metabolic Networks and Pathways / Q000235:genetics; D008958:Models, Molecular; D008969:Molecular Sequence Data; D020102:Mycobacterium smegmatis / Q000235:genetics* / Q000378:metabolism*; D011401:Promoter Regions, Genetic; D011487:Protein Conformation; D012097:Repressor Proteins / Q000737:chemistry / Q000378:metabolism*; D016415:Sequence Alignment; D024363:Transcription Initiation Site
chemical_list: D012097:Repressor Proteins; D002784:Cholesterol; D015096:3-Hydroxysteroid Dehydrogenases