REDUCTIVE PENTOSE PHOSPHATE-INDEPENDENT CO2 FIXATION IN RHODOBACTER-SPHAEROIDES AND EVIDENCE THAT RIBULOSE-BISPHOSPHATE CARBOXYLASE OXYGENASE ACTIVITY SERVES TO MAINTAIN THE REDOX BALANCE OF THE CELL

Whole-cell CO2 fixation and ribulose 1,5-bisphosphate carboxylase/oxyg enase (RubisCO) activity were determined in Rhodobacter sphaeroides wi ld-type and mutant strains. There is no obvious difference in the leve ls of whole-cell CO2 fixation for the wild type, a form I RubisCO dele tion mutant, and a form II RubisCO deletion mutant. No ribulose 1,5-bi sphosphate-dependent CO2 fixation was detected in a form I-form II Rub isCO double-deletion mutant (strain 16) or strain 16PHC, a derivative from strain 16 which was selected for the ability to grow photoheterot rophically with CO2 as an electron acceptor. However, significant leve ls of whole-cell CO2 fixation were detected in both strains 16 and 16P HC. Strain 16PHC exhibited CO2 fixation rates significantly higher tha n those of strain 16; the rates found for strain 16PHC were 30% of the level found in photoheterotrophically grown wild-type strain HR conta ining both form I and form II RubisCO and 10% of the level of the wild -type strain grown photolithoautotrophically. Strain 16PHC could not g row photolithoautotrophically in a CO2-H-2 atmosphere; however, CO2 fi xation catalyzed by photoheterotrophically grown strain 16PHC was repr essed by addition of the alternate electron acceptor dimethyl sulfoxid e. Dimethyl sulfoxide addition also influenced RubisCO activity under photolithoautotrophic conditions; 40 to 70% of the RubisCO activity wa s reduced without significantly influencing growth. Strain 16PHC and s train 16 contain nearly equivalent but low levels of pyruvate carboxyl ase, indicating that CO2 fixation enzymes other than pyruvate carboxyl ase contribute to the ability of strain 16PHC to grow with CO2 as an e lectron acceptor.