The insertion of a mitochondrial selfish element into the nuclear genome and its consequences

Abstract

Homing endonucleases (HE) are enzymes capable of cutting DNA at highly specific target sequences, the repair of the generated double‐strand break resulting in the insertion of the HE‐encoding gene (“homing” mechanism). HEs are present in all three domains of life and viruses; in eukaryotes, they are mostly found in the genomes of mitochondria and chloroplasts, as well as nuclear ribosomal RNAs. We here report the case of a HE that accidentally integrated into a telomeric region of the nuclear genome of the fungal maize pathogen Ustilago maydis . We show that the gene has a mitochondrial origin, but its original copy is absent from the U. maydis mitochondrial genome, suggesting a subsequent loss or a horizontal transfer from a different species. The telomeric HE underwent mutations in its active site and lost its original start codon. A potential other start codon was retained downstream, but we did not detect any significant transcription of the newly created open reading frame, suggesting that the inserted gene is not functional. Besides, the insertion site is located in a putative RecQ helicase gene, truncating the C‐terminal domain of the protein. The truncated helicase is expressed during infection of the host, together with other homologous telomeric helicases. This unusual mutational event altered two genes: The integrated HE gene subsequently lost its homing activity, while its insertion created a truncated version of an existing gene, possibly altering its function. As the insertion is absent in other field isolates, suggesting that it is recent, the U. maydis 521 reference strain offers a snapshot of this singular mutational event.

First Characterization of a Hafnia Phage Reveals Extraordinarily Large Burst Size and Unusual Plaque Polymorphism

“…Bioinformatic analysis of Kala et al (2014) revealed that the role of HNH proteins in terminase function is widespread among long-tailed phages and is uniquely required for the activity of the Terminase1 family of large terminase proteins. HEGs, encoding endonucleases capable of cutting site-specific DNA that promotes the lateral transfer of their own, can themselves be mobile elements moving independently ( Mota and Collins, 1988 ; Stoddard, 2005 ; Buttimer et al, 2020 ; Dutheil et al, 2020 ). Mobile genetic elements have the potential to influence the expression of genes surrounding their insertion site upon invasion of a genome ( Gibb and Edgell, 2007 ).…”

Global Gene Expression of Post-Senescent Telomerase-Negative ter1Δ Strain of Ustilago maydis

“…UTASb is near telomere repeats or 5’ upstream of UTASa. UTASb may be a truncated segment generated from the homing endonuclease gene HEG [ 43 ]. UTASb consists of a highly conserved “core” sequence, an OrsD domain, or both [ 42 ].…”