Three separate mitochondrial DNA sequences are contiguous in human genomic DNA

Area of Science:

• Genetics
• Molecular Biology
• Genomics

Background:

• Mitochondrial DNA (mtDNA) plays a crucial role in cellular respiration and is maternally inherited.
• The presence of mtDNA sequences in the nuclear genome (nuclear copies of mitochondrial DNA or NUMTs) has been reported in various organisms.
• Understanding the origin, integration, and evolutionary significance of NUMTs is important for genomic studies.

Purpose of the Study:

• To isolate and characterize nuclear DNA fragments that hybridize with human mitochondrial DNA.
• To determine the location and similarity of these nuclear mtDNA-like sequences to known human mtDNA genes.
• To investigate the evolutionary history and species specificity of these nuclear mtDNA-like sequences.

Main Methods:

• Screening of a HeLa genomic DNA library using total human placental mitochondrial DNA as a probe.
• Detailed characterization of a positive clone (HLmt-17.8) using DNA sequencing.
• Comparative sequence analysis to determine similarity to human mtDNA genes.
• Detection of mtDNA-like sequences in DNA from various human tissues and cell lines, as well as other species.

Main Results:

• Isolation of 38 plaques hybridizing to human mtDNA, with one clone (HLmt-17.8) containing a 17.8 kb insert.
• Identification of a 1.6 kb fragment within HLmt-17.8 showing high similarity (92.0-92.4%) to human mtDNA genes (12S rRNA, cytochrome oxidase I, ND4L/ND4).
• Detection of these mtDNA-like sequences in human DNAs but not in other primates or lower organisms, indicating recent integration specific to the human lineage.
• The HLmt-17.8 mtDNA-like region is proposed to be a nuclear pseudogene transferred to the nucleus within the last nine million years.

Conclusions:

• The human nuclear genome contains integrated mtDNA-like sequences, representing the highest similarity reported to date.
• These sequences are specific to humans and suggest a recent transfer event from the mitochondrial genome to the nucleus.
• The characterized mtDNA-like region in HLmt-17.8 functions as a pseudogene, providing insights into nuclear-mitochondrial DNA interactions and genome evolution in humans.