Abstract
Although the large majority of mitochondrial proteins are nuclear encoded, for their correct functioning mitochondria require the expression of 13 proteins, two rRNA, and 22 tRNA codified by mitochondrial DNA (mtDNA). Once transcribed, mitochondrial RNA (mtRNA) is processed, mito-ribosomes are assembled, and mtDNA-encoded proteins belonging to the respiratory chain are synthesized. These processes require the coordinated spatio-temporal action of several enzymes, and many different factors are involved in the regulation and control of protein synthesis and in the stability and turnover of mitochondrial RNA. In this review, we describe the essential steps of mitochondrial RNA synthesis, maturation, and degradation, the factors controlling these processes, and how the alteration of these processes is associated with human pathologies.
Highlights
Given its endosymbiotic bacterial origins, it is not surprising that the organization of DNA in mitochondria is similar to that of bacterial DNA
The bacterial genome is compacted by a factor of 104-fold that of its volume to form the bacterial nucleoid, and in a similar way the mitochondrial DNA is compacted and organized in discrete protein–DNA complexes distributed throughout the mitochondrial matrix [1]. mtDNA was first described in the 1960s [2] and completely sequenced in 1981 by Anderson et al [3]
It is in the non-coding region of the D-loop that mitochondrial translation is regulated by the heavy strand promoters 1 and 2 (HSP1 and HSP2), the light strand promoter (LSP) [17], the transcription termination-associated sequences TAS1 and TAS2, and the conserved sequence blocks1 and 2 (CSB1 and conserved sequence block 2 (CSB2))
Summary
Given its endosymbiotic bacterial origins, it is not surprising that the organization of DNA in mitochondria is similar to that of bacterial DNA. The bacterial genome is compacted by a factor of 104-fold that of its volume to form the bacterial nucleoid, and in a similar way the mitochondrial DNA (mtDNA) is compacted and organized in discrete protein–DNA complexes distributed throughout the mitochondrial matrix [1]. There are several unique features of the mtDNA: human mtDNA is circular, 16 kbp long, and inherited from the mother. It encodes two rRNAs, 22 tRNAs, and 13 proteins, all of which are involved in the oxidative phosphorylation process [4]. Quantitative analysis of the size and mtDNA content of the nucleoid in cultured mammalian cells suggests that an average nucleoid may contain five to seven mtDNA molecules packed in a space of 70 nm [6], similar to the bacterial genome. Tight packaging of the mtDNA is achieved thanks to the proteins present in the nucleoid, such as the mitochondrial transcription factor TFAM
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