Roles of RubisCO and the RubisCO-Like Protein in 5-Methylthioadenosine Metabolism in the Nonsulfur Purple Bacterium Rhodospirillum rubrum

ABSTRACT Ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO) catalyzes the assimilation of atmospheric CO2 into organic matter and is thus central to the existence of life on earth. The beginning of the 2000s was marked by the discovery of a new family of proteins, the RubisCO-like proteins (RLPs), which are structural homologs of RubisCO. RLPs are unable to catalyze CO2 fixation. The RLPs from Chlorobaculum tepidum, Bacillus subtilis, Geobacillus kaustophilus, and Microcystis aeruginosa have been shown to participate in sulfur metabolism. Whereas the precise function of C. tepidum RLP is unknown, the B. subtilis, G. kaustophilus, and M. aeruginosa RLPs function as tautomerases/enolases in a methionine salvage pathway (MSP). Here, we show that the form II RubisCO enzyme from the nonsulfur purple bacterium Rhodospirillum rubrum is also able to function as an enolase in vivo as part of an MSP, but only under anaerobic conditions. However, unlike B. subtilis RLP, R. rubrum RLP does not catalyze the enolization of 2,3-diketo-5-methylthiopentyl-1-phosphate. Instead, under aerobic growth conditions, R. rubrum RLP employs another intermediate of the MSP, 5-methylthioribulose-1-phosphate, as a substrate, resulting in the formation of different products. To further determine the interrelationship between RubisCOs and RLPs (and the potential integration of cellular carbon and sulfur metabolism), the functional roles of both RubisCO and RLP have been examined in vivo via the use of specific knockout strains and complementation studies of R. rubrum. The presence of functional, yet separate, MSPs in R. rubrum under both aerobic (chemoheterotrophic) and anaerobic (photoheterotrophic) growth conditions has not been observed previously in any organism. Moreover, the aerobic and anaerobic sulfur salvage pathways appear to be differentially controlled, with novel and previously undescribed steps apparent for sulfur salvage in this organism.

Conjugative Transfer and Expression of Genes Coding for Periplasmic Nitrate Reductase in the Purple Bacterium Rhodospirillum rubrum

Plasmid pFR400, a derivative of the vector plasmid pPHU231 containing the structural genes of the periplasmic nitrate reductase (nap genes) of Rhodobacter sphaeroides DSM 158 (F. Reyes et al., Molec. Microbiol. 19, 1307-1318 [1996]) was transferred by conjugative mating to a streptomycin-resistant strain of the nitrate reductase-negative nonsulfur purple bacterium Rhodospirillum rubrum S1. Transconjugant cells of the latter bacterium, identified by their resistance to streptomycin and tetracycline and by their plasmid pattern, contained an active nitrate reductase which, like other periplasmic bacterial nitrate reductases, was not repressed by ammonium.