A concert of RNA-binding proteins coordinates mitochondrial function

Recent findings demonstrate that multiple mRNAs are co-regulated by one or more sequence-specific RNA-binding proteins that orchestrate their splicing, export, stability, localization and translation. These and other observations have given rise to a model in which mRNAs that encode functionally related proteins are coordinately regulated during cell growth and differentiation as post-transcriptional RNA operons or regulons, through a ribonucleoprotein-driven mechanism. Here I describe several recently discovered examples of RNA operons in budding yeast, fruitfly and mammalian cells, and their potential importance in processes such as immune response, oxidative metabolism, stress response, circadian rhythms and disease. I close by considering the evolutionary wiring and rewiring of these combinatorial post-transcriptional gene-expression networks.

Mammalian mitochondrial DNA (mtDNA) encodes 13 proteins that are essential for the function of the oxidative phosphorylation system, which is composed of four respiratory-chain complexes and adenosine triphosphate (ATP) synthase. Remarkably, the maintenance and expression of mtDNA depend on the mitochondrial import of hundreds of nuclear-encoded proteins that control genome maintenance, replication, transcription, RNA maturation, and mitochondrial translation. The importance of this complex regulatory system is underscored by the identification of numerous mutations of nuclear genes that impair mtDNA maintenance and expression at different levels, causing human mitochondrial diseases with pleiotropic clinical manifestations. The basic scientific understanding of the mechanisms controlling mtDNA function has progressed considerably during the past few years, thanks to advances in biochemistry, genetics, and structural biology. The challenges for the future will be to understand how mtDNA maintenance and expression are regulated and to what extent direct intramitochondrial cross talk between different processes, such as transcription and translation, is important.

The composition of cytoplasmic messenger ribonucleoproteins (mRNPs) is determined by their nuclear and cytoplasmic histories and reflects past functions and future fates. The protein components of selected mRNP complexes promote their assembly into microscopically visible cytoplasmic RNA granules, including stress granules, processing bodies and germ cell (or polar) granules. We propose that RNA granules can be both a cause and a consequence of altered mRNA translation, decay or editing. In this capacity, RNA granules serve as key modulators of post-transcriptional and epigenetic gene expression.