A Bacterial Form I’ Rubisco Has a Smaller Carbon Isotope Fractionation than Its Form I Counterpart

Abstract

Form I rubiscos evolved in Cyanobacteria ≥ 2.5 billion years ago and are enzymatically unique due to the presence of small subunits (RbcS) capping both ends of an octameric large subunit (RbcL) rubisco assembly to form a hexadecameric (L8S8) holoenzyme. Although RbcS was previously thought to be integral to Form I rubisco stability, the recent discovery of a closely related sister clade of octameric rubiscos (Form I’; L8) demonstrates that the L8 complex can assemble without small subunits (Banda et al. 2020). Rubisco also displays a kinetic isotope effect (KIE) where the 3PG product is depleted in 13C relative to 12C. In Cyanobacteria, only two Form I KIE measurements exist, making interpretation of bacterial carbon isotope data difficult. To aid comparison, we measured in vitro the KIEs of Form I’ (Candidatus Promineofilum breve) and Form I (Synechococcus elongatus PCC 6301) rubiscos and found the KIE to be smaller in the L8 rubisco (16.25 ± 1.36‰ vs. 22.42 ± 2.37‰, respectively). Therefore, while small subunits may not be necessary for protein stability, they may affect the KIE. Our findings may provide insight into the function of RbcS and allow more refined interpretation of environmental carbon isotope data.

Carbon isotope fractionation by an ancestral rubisco suggests that biological proxies for CO 2 through geologic time should be reevaluated

“…We measured the carbon isotope fractionations of WT and ANC rubiscos in vitro using the substrate depletion method ( 48 – 51 ). Note that there exists experimental variation in ε Rubisco measurements, both within and across studies, and its cause remains uncertain at present [see SI Appendix , section 4a and ( 52 )]; so we employed the same general approach as others (the substrate depletion method) to be consistent with prior literature. Previous work on rubisco isotope discrimination predicted that ε Rubsico should correlate positively with specificity (S C/O ), a unitless measure of the relative preference for CO 2 over O 2 ( 53 ).…”