A novel RNA binding protein affects rbc L gene expression and is specific to bundle sheath chloroplasts in C 4 plants

Background Plants that utilize the highly efficient C 4 pathway of photosynthesis typically possess kranz-type leaf anatomy that consists of two morphologically and functionally distinct photosynthetic cell types, the bundle sheath (BS) and mesophyll (M) cells. These two cell types differentially express many genes that are required for C 4 capability and function. In mature C 4 leaves, the plastidic rbc L gene, encoding the large subunit of the primary CO 2 fixation enzyme Rubisco, is expressed specifically within BS cells. Numerous studies have demonstrated that BS-specific rbc L gene expression is regulated predominantly at post-transcriptional levels, through the control of translation and mRNA stability. The identification of regulatory factors associated with C 4 patterns of rbc L gene expression has been an elusive goal for many years. Results RLSB, encoded by the nuclear RLSB gene, is an S1-domain RNA binding protein purified from C 4 chloroplasts based on its specific binding to plastid-encoded rbc L mRNA in vitro . Co-localized with LSU to chloroplasts, RLSB is highly conserved across many plant species. Most significantly, RLSB localizes specifically to leaf bundle sheath (BS) cells in C 4 plants. Comparative analysis using maize (C 4 ) and Arabidopsis (C 3 ) reveals its tight association with rbc L gene expression in both plants. Reduced RLSB expression (through insertion mutation or RNA silencing, respectively) led to reductions in rbc L mRNA accumulation and LSU production. Additional developmental effects, such as virescent/yellow leaves, were likely associated with decreased photosynthetic function and disruption of associated signaling networks. Conclusions Reductions in RLSB expression, due to insertion mutation or gene silencing, are strictly correlated with reductions in rbc L gene expression in both maize and Arabidopsis . In both plants, accumulation of rbc L mRNA as well as synthesis of LSU protein were affected. These findings suggest that specific accumulation and binding of the RLSB binding protein to rbc L mRNA within BS chloroplasts may be one determinant leading to the characteristic cell type-specific localization of Rubisco in C 4 plants. Evolutionary modification of RLSB expression, from a C 3 “default” state to BS cell-specificity, could represent one mechanism by which rbc L expression has become restricted to only one cell type in C 4 plants.