title: Programmed Ribosomal Frameshifting Mediates Expression of the α-Carboxysome.
abstract: Many bacteria employ a protein organelle, the carboxysome, to catalyze carbon dioxide fixation in the Calvin Cycle. Only 10 genes from Halothiobacillus neapolitanus are sufficient for heterologous expression of carboxysomes in Escherichia coli, opening the door to detailed mechanistic analysis of the assembly process of this complex (more than 200MDa). One of these genes, csoS2, has been implicated in assembly but ascribing a molecular function is confounded by the observation that the single csoS2 gene yields expression of two gene products and both display an apparent molecular weight incongruent with the predicted amino acid sequence. Here, we elucidate the co-translational mechanism responsible for the expression of the two protein isoforms. Specifically, csoS2 was found to possess -1 frameshifting elements that lead to the production of the full-length protein, CsoS2B, and a truncated protein, CsoS2A, which possesses a C-terminus translated from the alternate frame. The frameshifting elements comprise both a ribosomal slippery sequence and a 3' secondary structure, and ablation of either sequence is sufficient to eliminate the slip. Using these mutants, we investigated the individual roles of CsoS2B and CsoS2A on carboxysome formation. In this in vivo formation assay, cells expressing only the CsoS2B isoform were capable of producing intact carboxysomes, while those with only CsoS2A were not. Thus, we have answered a long-standing question about the nature of CsoS2 in this model microcompartment and demonstrate that CsoS2B is functionally distinct from CsoS2A in the assembly of α-carboxysomes.
authors.affiliation: Department of Chemistry, University of California Berkeley, Berkeley, CA 94720, USA.
authors.forename: Thawatchai
authors.initials: T
authors.lastname: Chaijarasphong
authors.affiliation: Department of Molecular and Cell Biology, University of California Berkeley, Berkeley, CA 94720, USA.
authors.forename: Robert J
authors.initials: RJ
authors.lastname: Nichols
authors.affiliation: Department of Molecular and Cell Biology, University of California Berkeley, Berkeley, CA 94720, USA.
authors.forename: Kaitlyn E
authors.initials: KE
authors.lastname: Kortright
authors.affiliation: Department of Molecular and Cell Biology, University of California Berkeley, Berkeley, CA 94720, USA.
authors.forename: Charlotte F
authors.initials: CF
authors.lastname: Nixon
authors.affiliation: Department of Molecular and Cell Biology, University of California Berkeley, Berkeley, CA 94720, USA.
authors.forename: Poh K
authors.initials: PK
authors.lastname: Teng
authors.affiliation: Department of Molecular and Cell Biology, University of California Berkeley, Berkeley, CA 94720, USA.
authors.forename: Luke M
authors.initials: LM
authors.lastname: Oltrogge
authors.affiliation: Department of Chemistry, University of California Berkeley, Berkeley, CA 94720, USA; Department of Molecular and Cell Biology, University of California Berkeley, Berkeley, CA 94720, USA; Energy Biosciences Institute, University of California Berkeley, Berkeley, CA 94720, USA. Electronic address: savage@berkeley.edu.
authors.forename: David F
authors.initials: DF
authors.lastname: Savage
mesh_terms: D001426:Bacterial Proteins / Q000096:biosynthesis* / Q000235:genetics*; D004926:Escherichia coli / Q000235:genetics; D018965:Frameshifting, Ribosomal; D015964:Gene Expression Regulation, Bacterial; D044222:Halothiobacillus / Q000235:genetics*; D046911:Macromolecular Substances / Q000378:metabolism; D020033:Protein Isoforms / Q000096:biosynthesis* / Q000235:genetics*; D055503:Protein Multimerization
chemical_list: D001426:Bacterial Proteins; D046911:Macromolecular Substances; D020033:Protein Isoforms
keywords: CsoS2; bacterial microcompartments; carboxysome; co-translational regulation