Bacterial Form I' rubisco has smaller carbon isotope fractionation than its Form I counterpart

Form I rubiscos evolved in Cyanobacteria ≥2.5 billion years ago and are enzymatically unique due to the presence of small subunits (RbcS) that cap 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 enzyme complex assembles 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.