Bacterial Rubisco supports the photosynthetic growth of tobacco.

Abstract

Efforts to substitute foreign Rubiscos into the plastids of higher plants hitherto have been unsuccessful. Either the foreign Rubiscos were unable to fold and assemble properly in the plastid or incompatibility between the foreign large subunits and the host¿s small subunits crippled the enzyme so that it could not support growth. Here we report the use of plastid transformation to replace the Rubisco of tobacco with the simple L2 dimer from the bacterium, Rhodospirillum rubrum. Although the kinetic properties of R. rubrum Rubisco are not suited to higher plant chloroplasts, they are sufficient to sustain full photosynthetic and reproductive growth of the transformed plants at high atmospheric CO2 concentrations. A transforming plasmid containing the promoter, 5¿-UTR and the first 42 coding nucleotides of tobacco rbcL fused to a bicistronic operon containing the R. rubrum rbcL coding region and a promoterless aadA gene was targeted to replace the tobacco rbcL in the large single copy region of the tobacco plastome. Following microprojectile bombardment of tobacco leaves, transformed plant tissue was only obtained when regenerated was carried out on selective media in atmospheres containing 5% CO2. Leaf gas-exchange measurements made on independent homoplasmic transformants are consistent with the content and kinetic properties of R. rubrum Rubisco. A detailed analysis of growth and biochemical characteristics of T1 `tobacco-rubrum¿ transformants will be presented. This achievement demonstrates that the activity of a Rubisco from an entirely different phylogeny can be integrated into plastid photosynthetic metabolism without prohibitive problems.