DNA methylation and Mycoplasma genomes

Area of Science:

• Genomics
• Molecular Evolution
• Bioinformatics

Background:

• CpG dinucleotides are underrepresented in many genomes.
• The DNA methylation hypothesis suggests this deficiency results from methylation-mediated cytosine deamination.
• Recent studies have challenged the methylation hypothesis, citing exceptions like Mycoplasma genitalium.

Purpose of the Study:

• To re-evaluate the DNA methylation hypothesis for CpG deficiency using comparative genomics.
• To investigate the evolutionary history of CpG deficiency in Mycoplasma species.

Main Methods:

• Comparative genomic analysis of Mycoplasma genitalium, Mycoplasma pneumoniae, Mycoplasma pulmonis, Ureaplasma urealyticum, and Staphylococcus aureus.
• Phylogenetic analysis to infer the presence and loss of CpG-specific methyltransferases.
• Quantification of CpG dinucleotide frequencies across the analyzed genomes.

Main Results:

• Genomic data are consistent with the DNA methylation hypothesis.
• An ancestral Mycoplasma species likely possessed CpG-specific methyltransferases, leading to strong CpG deficiency.
• Subsequent gene loss and differential evolutionary rates explain the varying CpG deficiency observed in extant species like M. pulmonis, M. pneumoniae, and M. genitalium.

Conclusions:

• The DNA methylation hypothesis remains a valid explanation for CpG deficiency.
• The evolutionary trajectory of methyltransferase genes and mutation rates explain observed variations in CpG content.
• Comparative genomics provides strong evidence for ancestral DNA methylation driving CpG underrepresentation.