Programmed Ribosomal Frameshifting Mediates Expression of the α-Carboxysome

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.