title: Evidence for direct binding between HetR from Anabaena sp. PCC 7120 and PatS-5.
abstract: HetR, master regulator of heterocyst differentiation in the filamentous cyanobacterium Anabaena sp. strain PCC 7120, stimulates heterocyst differentiation via transcriptional autoregulation and is negatively regulated by PatS and HetN, both of which contain the active pentapeptide RGSGR. However, the direct targets of PatS and HetN remain uncertain. Here, we report experimental evidence for direct binding between HetR and the C-terminal RGSGR pentapeptide, PatS-5. Strains with a hetR allele coding for conservative substitutions at residues 250-256 had altered patterns of heterocysts and, in some cases, reduced sensitivity to PatS-5. Cysteine scanning mutagenesis coupled with electron paramagnetic resonance (EPR) spectroscopy showed quenching of spin label motion at HetR amino acid 252 upon titration with PatS-5, indicating direct binding of PatS-5 to HetR. Gel shift assays indicated that PatS-5 disrupted binding of HetR to a 29 base pair inverted-repeat-containing DNA sequence upstream from hetP. Double electron-electron resonance EPR experiments confirmed that HetR existed as a dimer in solution and indicated that PatS-5 bound to HetR without disrupting the dimer form of HetR. Isothermal titration calorimetry experiments corroborated direct binding of PatS-5 to HetR with a K(d) of 227 nM and a 1:1 stoichiometry. Taken together, these results indicated that PatS-5 disrupted HetR binding to DNA through a direct HetR/PatS interaction. PatS-5 appeared to either bind in the vicinity of HetR amino acid L252 or, alternately, to bind in a remote site that leads to constrained motion of this amino acid via an allosteric effect or change in tertiary structure.
authors.affiliation: Department of Chemistry and Biochemistry, Miami University, Oxford, Ohio 45056, United States.
authors.forename: Erik A
authors.initials: EA
authors.lastname: Feldmann
authors.forename: Shuisong
authors.initials: S
authors.lastname: Ni
authors.forename: Indra D
authors.initials: ID
authors.lastname: Sahu
authors.forename: Clay H
authors.initials: CH
authors.lastname: Mishler
authors.forename: Douglas D
authors.initials: DD
authors.lastname: Risser
authors.forename: Jodi L
authors.initials: JL
authors.lastname: Murakami
authors.forename: Sasa K
authors.initials: SK
authors.lastname: Tom
authors.forename: Robert M
authors.initials: RM
authors.lastname: McCarrick
authors.forename: Gary A
authors.initials: GA
authors.lastname: Lorigan
authors.forename: Blanton S
authors.initials: BS
authors.lastname: Tolbert
authors.forename: Sean M
authors.initials: SM
authors.lastname: Callahan
authors.forename: Michael A
authors.initials: MA
authors.lastname: Kennedy
mesh_terms: D019943:Amino Acid Substitution; D017033:Anabaena / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D001426:Bacterial Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D004269:DNA, Bacterial / Q000378:metabolism; D008958:Models, Molecular; D009154:Mutation; D011485:Protein Binding; D055503:Protein Multimerization; D013816:Thermodynamics
chemical_list: D001426:Bacterial Proteins; D004269:DNA, Bacterial; C115380:PatS protein, Anabaena; C067403:HetR protein, Bacteria