Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is a rate-limiting photosynthetic enzyme that catalyzes carbon fixation in the Calvin cycle. Much interest has been devoted to engineering this ubiquitous enzyme with the goal of increasing plant growth. However, experiments that have successfully produced improved Rubisco variants, via directed evolution in Escherichia coli, are limited to bacterial Rubisco because the eukaryotic holoenzyme cannot be produced in E. coli. The present study attempts to determine the specific differences between bacterial and eukaryotic Rubisco large subunit primary structure that are responsible for preventing heterologous eukaryotic holoenzyme formation in E. coli. A series of chimeric Synechococcus Rubiscos were created in which different sections of the large subunit were swapped with those of the homologous Chlamydomonas Rubisco. Chimeric holoenzymes that can form in vivo would indicate that differences within the swapped sections do not disrupt holoenzyme formation. Large subunit residues 1-97, 198-247 and 448-472 were successfully swapped without inhibiting holoenzyme formation. In all ten chimeras, protein expression was observed for the separate subunits at a detectable level. As a first approximation, the regions that can tolerate swapping may be targets for future engineering.

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Engineering the CO 2-fixing enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) to improve photosynthesis has long been sought. Rubisco large subunits (RbcL) are highly-conserved but because of certain undefined sequence differences, plant Rubisco research cannot fully utilise the robust heterologous Escherichia coli expression system and its GroEL folding machinery. Previously, a series of chimeric cyanobacteria Synechococcus elongatus Rubisco, incorporated with sequences from the green alga Chlamydomonas reinhardtii, were expressed in E. coli; differences in RbcL sections essential for holoenzyme formation were pinpointed. In this study, the remaining sections, presumably not crucial for holoenzyme formation and also the small subunit (RbcS), are substituted to further ascertain the possible destabilising effects of multiple section mutations. To that end, combinations of Synechococcus RbcL Sections 1 (residues 1-47), 2 (residues 48-97), 5 (residues 198-247) and 10 (residues 448-472), and RbcS, were swapped with collinear Chlamydomonas sections and expressed in E. coli. Interestingly, only the chimera with Sections 1 and 2 together produces holoenzyme and an interaction network of complementing amino acid changes is delineated by crystal structure analysis. Furthermore, sequence-based analysis also highlighted possible GroEL binding site differences between the two RbcLs.