CO2-Responsive Expression and Gene Organization of Three Ribulose-1,5-Bisphosphate Carboxylase/Oxygenase Enzymes and Carboxysomes in Hydrogenovibrio marinus Strain MH-110

摘要

Hydrogenovibrio marinus strain MH-110, an obligately lithoautotrophic hydrogen-oxidizing bacterium, fixes CO₂ by the Calvin-Benson-Bassham cycle. Strain MH-110 possesses three different sets of genes for ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO): CbbLS-1 and CbbLS-2, which belong to form I (L₈S₈), and CbbM, which belongs to form II (L_x). In this paper, we report that the genes for CbbLS-1 (cbbLS-1) and CbbM (cbbM) are both followed by the cbbQO genes and preceded by the cbbR genes encoding LysR-type regulators. In contrast, the gene for CbbLS-2 (cbbLS-2) is followed by genes encoding carboxysome shell peptides. We also characterized the three RubisCOs in vivo by examining their expression profiles in environments with different CO₂ availabilities. Immunoblot analyses revealed that when strain MH-110 was cultivated in 15% CO₂, only the form II RubisCO, CbbM, was expressed. When strain MH-110 was cultivated in 2% CO₂, CbbLS-1 was expressed in addition to CbbM. In the 0.15% CO₂ culture, the expression of CbbM decreased and that of CbbLS-1 disappeared, and CbbLS-2 was expressed. In the atmospheric CO₂ concentration of approximately 0.03%, all three RubisCOs were expressed. Transcriptional analyses of mRNA by reverse transcription-PCR showed that the regulation was at the transcriptional level. Electron microscopic observation of MH-110 cells revealed the formation of carboxysomes in the 0.15% CO₂ concentration. The results obtained here indicate that strain MH-110 adapts well to various CO₂ concentrations by using different types of RubisCO enzymes.