AEROBIC CHEMOLITHOAUTOTROPHIC GROWTH AND RUBISCO FUNCTION IN RHODOBACTER-CAPSULATUS AND A SPONTANEOUS GAIN OF FUNCTION MUTANT OF RHODOBACTER-SPHAEROIDES

Photosynthetic prokaryotes that assimilate CO2 under anoxic conditions may also grow chemolithoautotrophically with O-2 as the electron acce ptor. Among the nonsulfur purple bacteria, two species (Rhodobacter ca psulatus and Rhodopseudomonas acidophilus), exhibit aerobic chemolitho autotrophic growth with hydrogen as the electron donor. Although wild- type strains of Rhodobacter sphaeroides grow poorly, if at all, with h ydrogen plus oxygen in the dark, we report here the isolation of a spo ntaneous mutant (strain HR-CAC) of Rba. sphaeroides strain HR that is fully capable of this mode of growth. Rba. sphaeroides and Rba. capsul atus fix CO2 via the reductive pentose phosphate pathway and synthesiz e two forms of ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisC O). RubisCO levels in the aerobic-chemolithoautotrophic-positive strai n of Rba. sphaeroides were similar to those in wild-type strains of Rb a. sphaeroides and Rba. capsulatus during photoheterotrophic and photo lithoautotrophic growth. Moreover, RubisCO levels of Rba. sphaeroides strain HR-CAC approximated levels obtained in Rba. capsulatus when the organisms were grown as aerobic chemolithoautotrophs. Either form I o r form II RubisCO was able to support aerobic chemolithoautotrophic gr owth of Rba. capsulatus strain SE 1003 and Rba. sphaeroides strain HR- CAC at a variety of CO2 concentrations, although form II RubisCO began to lose the capacity to support aerobic CO2 fixation at high O-2 to C O2 ratios. The latter property and other facets of the physiology of t his system suggest that Rba. sphaeroides and Rba. capsulatus strains m ay be effectively employed for the biological selection of RubisCO mol ecules of altered substrate specificity.