Figure 2. Nuclear anterograde control of organelle gene expression.

A generalized model of the coordination of organelle gene expression through nuclear anterograde control in eukaryotes. Several processes are highlighted. a | In flowering plants, restorers of fertility (Rf) proteins reverse the cytoplasmic male sterility (CMS) phenotype by reducing the expression of the aberrant mitochondrial gene using post-transcriptional mechanisms, b | Post-translational control of mitochondrial gene expression using nuclear-encoded proteases and assembly proteins, c | Post-transcriptional control of chloroplast gene expression using nuclear-encoded regulators of organelle gene expression (ROGE) proteins that target specific RNA transcripts, d | Control by epistasy of synthesis (CES) is an autoregulatory process in which unassembled organelle proteins repress their own translation. The autoregulation of the petA gene encoding the cytochrome f subunit (Cyt f) of the cytochrome b₆f complex in Chlamydomonas reinhardtii is used as an example of the CES process. In scenario 1, unassembled Cyt f binds the ROGE protein translation factor TCA1, and translation of the petA mRNA is inhibited. In scenario 2, the presence of cognate protein subunits cytochrome b₆ (Cyt b₆) and cytochrome subunit IV (SUIV) assemble with Cyt f. TCA1 is now able to bind petA mRNA and activate its translation. A discussion of which proteins are organelle encoded can be found in BOX 1. A more complete model of retrograde signalling than that highlighted in panel a can be viewed in FIG. 3. Proteins and protein complexes are designated as ovals. PPR pentatricopeptide repeat; TPR, tetratricopeptide repeat.