Function of three RuBisCO enzymes under different CO2 conditions in Hydrogenovibrio marinus

Abstract

The obligate chemolithoautotrophic bacterium, Hydrogenovibrio marinus MH-110 has three ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) isoenzymes, designated CbbLS-1, CbbLS-2, and CbbM, which are encoded by the cbbL1S1, cbbL2S2, and cbbM genes, respectively. Functions of these isoenzymes at different CO₂ concentrations were investigated using deletion mutants of their genes. Deletion of cbbL1 had no effect on cell growth under any of the test growth conditions. The cbbL2 mutant was unable to grow under lower (≤0.15%) CO₂ conditions, though it grew normally under higher (≥2%) CO₂ conditions. Growth of the cbbM mutant was retarded under higher CO₂ conditions but was not affected by lower CO₂ conditions. These results indicate that CbbLS-2 and CbbM specifically function under lower and higher CO₂ conditions, respectively. The growth retardation of the cbbL2 and cbbM mutants was not restored by complementation with plasmids carrying the cbbL2S2 and cbbM genes, respectively. The cbbL2S2 and cbbM genes are followed by the carboxysome genes and the cbbQmOm genes, respectively. Co-expression of these downstream genes was probably necessary for the in vivo function of CbbLS-2 and CbbM. CbbLS-1 was upregulated in the cbbL2 and cbbM mutants under the lower and higher CO₂ conditions, respectively, indicating that the expression of cbbL1S1 was controlled to compensate the deficiency of the other RuBisCO isoenzymes.

Keywords: Calvin-Benson-Bassham cycle; Carbon dioxide fixation; Carboxysome; Chemolithoautotrophic bacterium; Hydrogenovibrio murinus; Ribulose-1,5-bisphosphate carboxylase/oxygenase.