Evolution of RLSB, a nuclear-encoded S1 domain RNA binding protein associated with post-transcriptional regulation of plastid-encoded rbcL mRNA in vascular plants

Background RLSB, an S-1 domain RNA binding protein of Arabidopsis , selectively binds rbc L mRNA and co-localizes with Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) within chloroplasts of C 3 and C 4 plants. Previous studies using both Arabidopsis (C 3 ) and maize (C 4 ) suggest RLSB homologs are post-transcriptional regulators of plastid-encoded rbc L mRNA. While RLSB accumulates in all Arabidopsis leaf chlorenchyma cells, in C 4 leaves RLSB-like proteins accumulate only within Rubisco-containing bundle sheath chloroplasts of Kranz-type species, and only within central compartment chloroplasts in the single cell C 4 plant Bienertia . Our recent evidence implicates this mRNA binding protein as a primary determinant of rbc L expression, cellular localization/compartmentalization, and photosynthetic function in all multicellular green plants. This study addresses the hypothesis that RLSB is a highly conserved Rubisco regulatory factor that occurs in the chloroplasts all higher plants. Results Phylogenetic analysis has identified RLSB orthologs and paralogs in all major plant groups, from ancient liverworts to recent angiosperms. RLSB homologs were also identified in algae of the division Charophyta , a lineage closely related to land plants. RLSB -like sequences were not identified in any other algae, suggesting that it may be specific to the evolutionary line leading to land plants. The RLSB family occurs in single copy across most angiosperms, although a few species with two copies were identified, seemingly randomly distributed throughout the various taxa, although perhaps correlating in some cases with known ancient whole genome duplications. Monocots of the order Poales (Poaceae and Cyperaceae) were found to contain two copies, designated here as RLSB- a and RLSB-b , with only RLSB-a implicated in the regulation of rbc L across the maize developmental gradient. Analysis of microsynteny in angiosperms revealed high levels of conservation across eudicot species and for both paralogs in ...