title: Characterization of mutations in the GTP-binding domain of IF2 resulting in cold-sensitive growth of Escherichia coli.
abstract: The infB gene encodes translation initiation factor IF2. We have determined the entire sequence of infB from two cold-sensitive Escherichia coli strains IQ489 and IQ490. These two strains have been isolated as suppressor strains for the temperature-sensitive secretion mutation secY24. The mutations causing the suppression phenotype are located within infB. The only variations from the wild-type (wt) infB found in the two mutant strains are a replacement of Asp409 with Glu in strain IQ489 and an insertion of Gly between Ala421 and Gly422 in strain IQ490. Both positions are located in the GTP-binding G-domain of IF2. A model of the G-domain of E.coli IF2 is presented in. Physiological quantities of the recombinant mutant proteins were expressed in vivo in E.coli strains from which the chromosomal infB gene has been inactivated. At 42 degrees C, the mutants sustained normal cell growth, whereas a significant decrease in growth rate was found at 25 degrees C for both mutants as compared to wt IF2 expressed in the control strain. Circular dichroism spectra were recorded of the wt and the two mutant proteins to investigate the structural properties of the proteins. The spectra are characteristic of alpha-helix dominated structure, and reveal a significant different behavior between the wt and mutant IF2s with respect to temperature-induced conformational changes. The temperature-induced conformational change of the wt IF2 is a two-state process. In a ribosome-dependent GTPase assay in vitro the two mutants showed practically no activity at temperatures below 10 degrees C and a reduced activity at all temperatures up to 45 degrees C, as compared to wt IF2. The results indicate that the amino acid residues, Asp409 and Gly422, are located in important regions of the IF2 G-domain and demonstrate the importance of GTP hydrolysis in translation initiation for optimal cell growth.
authors.affiliation: Department of Molecular Biology, Aarhus University, DK-8000 C Aarhus, Denmark.
authors.forename: Brian Søgaard
authors.initials: BS
authors.lastname: Laursen
authors.forename: Igor
authors.initials: I
authors.lastname: Siwanowicz
authors.forename: Guilhem
authors.initials: G
authors.lastname: Larigauderie
authors.forename: Jakob
authors.initials: J
authors.lastname: Hedegaard
authors.forename: Koreaki
authors.initials: K
authors.lastname: Ito
authors.forename: Yoshikazu
authors.initials: Y
authors.lastname: Nakamura
authors.forename: John M
authors.initials: JM
authors.lastname: Kenney
authors.forename: Kim Kusk
authors.initials: KK
authors.lastname: Mortensen
authors.forename: Hans Uffe
authors.initials: HU
authors.lastname: Sperling-Petersen
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000378:metabolism*; D001483:Base Sequence; D001665:Binding Sites; D002942:Circular Dichroism; D003001:Cloning, Molecular; D003080:Cold Temperature; D004926:Escherichia coli / Q000235:genetics / Q000254:growth & development* / Q000378:metabolism*; D020558:GTP Phosphohydrolases / Q000378:metabolism; D015964:Gene Expression Regulation, Bacterial; D005816:Genetic Complementation Test; D006160:Guanosine Triphosphate / Q000378:metabolism; D066298:In Vitro Techniques; D008958:Models, Molecular; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D009154:Mutation; D010957:Plasmids; D039881:Prokaryotic Initiation Factor-2 / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D011487:Protein Conformation; D015202:Protein Engineering; D011994:Recombinant Proteins / Q000096:biosynthesis / Q000737:chemistry; D015183:Restriction Mapping
chemical_list: D001426:Bacterial Proteins; D039881:Prokaryotic Initiation Factor-2; D011994:Recombinant Proteins; D006160:Guanosine Triphosphate; D020558:GTP Phosphohydrolases