Photosynthesis and growth of tobacco with a substituted bacterial Rubisco mirror the properties of the introduced enzyme

Complete replacement, by biolistic plastid transformation, of the hexadecameric ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) of tobacco (Nicotiana tabacum) with the dimeric version from the bacterium, Rhodospirillum rubrum, resulted in fully autotrophic and reproductive tobacco plants that required high CO₂ concentrations to grow (Whitney SM, Andrews TJ [2001] Proc Natl Acad Sci USA 98: 14738-14743). Growth and photosynthesis of these plants was compared with that of nontransformed tobacco and other controls where the rbcL gene for the large subunit of tobacco Rubisco was linked to the aadA selectable-marker gene, simulating the gene arrangement of the transformants with R. rubrum Rubisco. An arrangement of the rbcL and aadA genes that gave rise to an abundant monocistronic rbcL transcript and a one-fifth as abundant bicistronic rbcL-aadA transcript had Rubisco levels and photosynthetic properties similar to those of nontransformed tobacco. Direct linkage of the rbcL and aadA genes, resulting in exclusive production of a bicistronic mRNA transcript analogous to that of the transformants with R. rubrum Rubisco, reduced transcript abundance and tobacco Rubisco content. The analogous transcript with the R. rubrum rbcM gene substituted for rbcL was not only reduced in abundance, but was also translated less efficiently. The photosynthetic rates of the transformants and controls were measured at high CO₂ concentrations, using a mass spectrometric method. The rates and their responses to atmospheric CO₂ concentration mirrored the amounts and the kinetic properties of the Rubiscos present. The contents of total nitrogen, carbohydrates, and photosynthetic metabolites of the leaves were also consistent with the content and type of Rubisco.