Origins of Genome-editing Excisases As Illuminated by the Somatic Genome of the Ciliate

Overview

Journal Proc Natl Acad Sci U S A

Specialty Science

Date 2023 Jan 20

PMID 36669098

Authors

Minakshi Singh

Brandon K B Seah

Christiane Emmerich

Aditi Singh

Christian Woehle

Bruno Huettel

Adam Byerly

Naomi A Stover

Mayumi Sugiura

Terue Harumoto

Estienne C Swart

Affiliations

Soon will be listed here.

Abstract

Massive DNA excision occurs regularly in ciliates, ubiquitous microbial eukaryotes with somatic and germline nuclei in the same cell. Tens of thousands of internally eliminated sequences (IESs) scattered throughout the ciliate germline genome are deleted during the development of the streamlined somatic genome. The genus represents one of the two high-level ciliate clades (subphylum Postciliodesmatophora) and, unusually, has dual pathways of somatic nuclear and genome development. This makes it ideal for investigating the functioning and evolution of these processes. Here we report the somatic genome assembly of strain ATCC 30299 (41 Mbp), arranged as numerous telomere-capped minichromosomal isoforms. This genome encodes eight PiggyBac transposase homologs no longer harbored by transposons All appear subject to purifying selection, but just one, the putative IES excisase, has a complete catalytic triad. We hypothesize that PiggyBac homologs were ancestral excisases that enabled the evolution of extensive natural genome editing.

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