Regulation of Mitochondrial Gene Expression
Transcription of the mitochondrial genome is unusual. Both strands of the mtDNA are almost completely transcribed from two oppositely oriented promoters within a region known as the D-loop, giving rise to polycistronic precursors which are then processed into mono- or bi-cistronic mRNAs, in most cases by excision of flanking tRNAs interspersed throughout the genome (the 'tRNA puctuation' model).
As with nuclear RNAs, mitochondrial RNAs are further modified during or after transcription by different processes including polyadenylation, methylation, aminoacylation or pseudouridinylation. However, in contrast to nuclear gene expression very little is known about the spatio-temporal organization of gene expression in mitochondria. In the absence of a clear membrane delineation within the mitochondrial matrix it is unclear how immature polycistronic RNAs are prevented from interacting with the translation machinery, where and how they are modified after transcription to generate mature transcripts, what governs RNA stability, and how and where the mitoribosomes assemble.
In 2012 our laboratory identified an RNA-rich subcompartment of mitochondria, which we termed MRGs (for Mitochondrial RNA Granules; Figure 1). These structures have subsequently been shown to contain many of the proteins involved in RNA processing, maturation and translation, and we believe that the MRGs represent key sites for the organization of mitochondrial gene expression.
Figure 1: Confocal analysis of HeLa cells showing the sub-mitochondrial distribution and co-localization of newly transcribed mitochondrial RNA labelled with BrU (red) and GRSF1-HA, a resident MRG protein (green). The mitochondrial network can be visualized by staining with mitochondria-localized Cyan Fluorescent Protein (blue). The lower panels show the boxed area at higher magnification.
The goals of our current research are,
- to characterize more completely the MRG composition using medium and high throughput methods with a view to extending our knowledge of their function;
- to investigate in greater detail the activities of selected proteins such as the FASTK family of proteins and others;
- to identify MRG assembly factors which may then allow us to disassemble the structures and assess their contribution to mitochondrial gene expression.