Fig. 7.
Proposed evolution pathways to carboxysomes from free Rubisco via condensation, before the advent of Ci transport. Model simulations propose condensation of Rubisco (blue/yellow) in the presence of a cellular CA enzyme (light pink), here presented as three possible evolutionary alternatives with the CA external (in the unstirred layer), internal, or both external and internal of the condensate. More detailed analysis shows that evolution of a Rubisco condensate in the absence of a CA is not feasible (Fig. 6). Condensation is proposed to occur through the evolution of a condensing protein factor (here, CcmM from β-carboxysomes, bright pink) and carboxysome formation via the acquisition of bacterial microcompartment shell proteins (gray, purple). Contemporary carboxysomes are represented by those containing only internal CA. Percent increase or decrease in average net carboxylation rates between each proposed evolutionary intermediate is indicated by the colored arrows, and the color scale indicates the values presented in Fig. 6. Between proposed evolutionary stages, yellow-shaded arrows indicate an improvement in average net carboxylation rate and blue-shaded arrows a net decrease, suggesting a loss in competitive fitness. No net change is indicated by a white arrow. The same pattern of potential evolutionary improvements is apparent regardless of the Rubisco source or carboxysome size used in the model, assuming sufficient RuBP supply (Datasets S1 and S2) (54). We assume the adaptation of increased cellular HCO3− followed as an evolutionary enhancement (14), hence the relative fitness of systems with CA external to the Rubisco compartment (here, modeled in the unstirred layer) where, in contemporary systems, this is problematic (49).