A mating-type mutagenesis screen identifies a zinc-finger protein required for specific DNA excision events in Paramecium

Nucleic Acids Res. 2018 Oct 12;46(18):9550-9562. doi: 10.1093/nar/gky772.


In the ciliate Paramecium tetraurelia, functional genes are reconstituted during development of the somatic macronucleus through the precise excision of ∼45 000 single-copy Internal Eliminated Sequences (IESs), thought to be the degenerate remnants of ancient transposon insertions. Like introns, IESs are marked only by a weak consensus at their ends. How such a diverse set of sequences is faithfully recognized and precisely excised remains unclear: specialized small RNAs have been implicated, but in their absence up to ∼60% of IESs are still correctly excised. To get further insight, we designed a mutagenesis screen based on the hypersensitivity of a specific excision event in the mtA gene, which determines mating types. Unlike most IES-containing genes, the active form of mtA is the unexcised one, allowing the recovery of hypomorphic alleles of essential IES recognition/excision factors. Such is the case of one mutation recovered in the Piwi gene PTIWI09, a key player in small RNA-mediated IES recognition. Another mutation identified a novel protein with a C2H2 zinc finger, mtGa, which is required for excision of a small subset of IESs characterized by enrichment in a 5-bp motif. The unexpected implication of a sequence-specific factor establishes a new paradigm for IES recognition and/or excision.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Alleles
  • DNA Repair / genetics*
  • DNA Transposable Elements / genetics
  • DNA, Protozoan / genetics
  • Gene Rearrangement / genetics
  • Introns / genetics
  • Macronucleus / genetics
  • Macronucleus / physiology
  • Mating Factor / genetics*
  • Mutagenesis / genetics*
  • Paramecium tetraurelia / genetics
  • Paramecium tetraurelia / physiology
  • RNA, Small Interfering / genetics
  • Reproduction / genetics*
  • Zinc Fingers / genetics*


  • DNA Transposable Elements
  • DNA, Protozoan
  • RNA, Small Interfering
  • Mating Factor