Divergent evolution of the paralogous human mitochondrial transcription factors, h-mtTFB1 and h-mtTFB2, to fulfill unique functions in mitochondrial gene expression, biogenesis, and retrograde signaling

<p>Mitochondria are essential organelles, resulting from an ancient endosymbiosis,that are found in virtually all eukaryotes. To maintain proper function, human mitochondria have maintained distinct genomes and a dedicated gene expression system. The human mitochondrial transcription system consists of three types of proteins: POLRMT, a bacteriophage-like RNA polymerase; h-mtTFA, a mitochondrial DNA binding protein; and the mtTFBs, two factors that provide a physical link between hmtTFA and POLRMT required for transcription initiation. The human mtTFBs, hmtTFB1 and h-mtTFB2, are unique transcription factors exhibiting homology with N6-adenine RNA dimethyltransferases. Phylogenetic analysis suggests a very early duplication of the endosymbiont KsgA gene as the source of these two paralogs. Consistent with this ancestry, I demonstrate that both h-mtTFB1 and h-mtTFB2 have maintained RNA methyltransferase activity. Overexpression of h-mtTFB1 in human cells increases the level of methylated 12S rRNA and induces mitochondrial mass. hmtTFB2 overexpression elevates mitochondrial DNA levels, transcripts, mass, membrane potential, and surprisingly induces a coordinate increase in h-mtTFB1 expression. This indicates that h-mtTFB1 is the major 12S methyltransferase and h-mtTFB2 is involved primarily in mitochondrial transcription. These results also suggest a major role for these factors in coordinating mitochondrial biogenesis. By combining their activities, a robust remodeling of the mitochondrial compartment occurs, increasing organelle mass and greater respiratory capacity. This response is dependent specifically upon methylation activity of h-mtTFB1 and indicates that the methylation status of 12S rRNA is a metric for mitochondrial function. In support of this idea cells harboring the mtDNA mutation A1555G, a mutation in 12S rRNA linked to non-syndromic deafness, have elevated methylated 12S rRNA and phenotypes similar to those associated with h-mtTFB1 overexpression. I propose that 12S rRNA methylation status is regularly monitored by the cell and either induces expression of h-mtTFB1 to improve overall methylation or induces mitochondrial mass in preparation for more OXPHOS complexes to be produced by fully methylated mitochondrial ribosomes. Altogether this research indicates that these factors are intimately involved in regulating mitochondrial biogenesis. Inappropriate modulation of their levels or activities might contribute to human disease by producing mitochondria deficient in transcription, translation, or respiration.</p> <p>