1.
Dear Editor:
In a recent and otherwise excellent review of mitochondrial transcription, Bouda et al.1 reiterated two misconceptions that exist in the mitochondrial literature. Due to the persistent nature of these misconceptions, I feel compelled to address them in this Letter.
First, the authors state, “The H‐strand denotes its high content in purine nucleotides compared to the L‐strand (owing mostly to a strong imbalance in the guanine/cytosine ratio), making the strands separable by using density centrifugation in alkaline CsCl2 gradients.” In fact, the two strands of mitochondrial DNA owe their differential buoyant densities in alkaline CsCl (not CsCl2) gradients to the differences in their G + T (rather than purine nucleotide) content. Indeed, it is believed that in alkaline solutions, G and T bases become ionized and can interact with Cs+ ions, thus conferring a higher density to the strand with a higher content of these bases.2
The authors also state that “very few genes are encoded in the L‐strand compared to the H‐strand.” As has been recently pointed out by us3 and others,4 the main mtDNA coding strand is, in fact, the light strand (L‐strand). The confusion in question dates back to 1981 when the first mitochondrial genome sequences were determined in humans and mice by Anderson et al. and Bibi et al., respectively.5, 6 Although these two mitochondrial genomes have essentially identical organization, the two groups arrived at opposite conclusions with respect to which mtDNA strand encodes the most genes. Anderson et al. stated “…L strand which we define as the main coding strand containing the sense sequence of the rRNAs and most of the tRNAs and mRNAs. Thus, these RNAs are transcribed from (and therefore hybridize to) the H strand”.5 Bibi et al. stated “The major coding strand of mouse mitochondrial DNA is the heavy (H) strand; most transcripts are therefore of light (L)‐ strand sequence and are complementary to the H strand.” Therefore, both groups agreed that the majority of mitochondrial transcripts in both humans and mice are identical in sequence to the L‐strand, which, by the current definition, makes the L‐strand the main mtDNA coding strand in these species.
Sincerely,
CONFLICT OF INTEREST
The author has no conflicts to declare.
ACKNOWLEDGMENTS
The authors wish to acknowledge support by NIH grants OD010944 and HL66299 and by the Office of the Assistant Secretary of Defense for Health Affairs under Award No. W81XWH‐16‐1‐0096. Opinions, interpretations, conclusions, and recommendations are those of the authors and are not necessarily endorsed by the Department of Defense.
Funding information National Heart, Lung, and Blood Institute, Grant/Award Number: HL66299; NIH Office of the Director, Grant/Award Number: OD010944; Office of the Assistant Secretary of Defense for Health Affairs, Grant/Award Number: W81XWH‐16‐1‐0096
REFERENCES
- 1. Bouda E, Stapon A, Garcia‐Diaz M. Mechanisms of mammalian mitochondrial transcription. Protein Sci. 2019;28:1594–1605. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 2. Wells RD, Larson JE. Buoyant density studies on natural and synthetic deoxyribonucleic acids in neutral and alkaline solutions. J Biol Chem. 1972;247:3405–3409. [PubMed] [Google Scholar]
- 3. Shokolenko IN, Alexeyev MF. Mitochondrial transcription in mammalian cells. Front Biosci. 2017;22:835–853. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 4. Barroso Lima NC, Prosdocimi F. The heavy strand dilemma of vertebrate mitochondria on genome sequencing age: Number of encoded genes or G + T content? Mitochondrial DNA A. 2018;29:300–302. [DOI] [PubMed] [Google Scholar]
- 5. Anderson S, Bankier AT, Barrell BG, et al. Sequence and organization of the human mitochondrial genome. Nature. 1981;290:457–465. [DOI] [PubMed] [Google Scholar]
- 6. Bibb MJ, Van Etten RA, Wright CT, Walberg MW, Clayton DA. Sequence and gene organization of mouse mitochondrial DNA. Cell. 1981;26:167–180. [DOI] [PubMed] [Google Scholar]