A Domesticated PiggyBac Transposase Interacts with Heterochromatin and Catalyzes Reproducible DNA Elimination in Tetrahymena

Kazufumi Mochizuki,Cédric Feschotte,Alexander Vogt

doi:10.1371/journal.pgen.1004032

Kazufumi Mochizuki, Cédric Feschotte + Show 1 more

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https://doi.org/10.1371/journal.pgen.1004032

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Abstract

The somatic genome of the ciliated protist Tetrahymena undergoes DNA elimination of defined sequences called internal eliminated sequences (IESs), which account for ∼30% of the germline genome. During DNA elimination, IES regions are heterochromatinized and assembled into heterochromatin bodies in the developing somatic nucleus. The domesticated piggyBac transposase Tpb2p is essential for the formation of heterochromatin bodies and DNA elimination. In this study, we demonstrate that the activities of Tpb2p involved in forming heterochromatin bodies and executing DNA elimination are genetically separable. The cysteine-rich domain of Tpb2p, which interacts with the heterochromatin-specific histone modifications, is necessary for both heterochromatin body formation and DNA elimination, whereas the endonuclease activity of Tpb2p is only necessary for DNA elimination. Furthermore, we demonstrate that the endonuclease activity of Tpb2p in vitro and the endonuclease activity that executes DNA elimination in vivo have similar substrate sequence preferences. These results strongly indicate that Tpb2p is the endonuclease that directly catalyzes the excision of IESs and that the boundaries of IESs are at least partially determined by the combination of Tpb2p-heterochromatin interaction and relaxed sequence preference of the endonuclease activity of Tpb2p.

Highlights

Transposons represent harmful genetic elements because they potentially rearrange their host’s genome, and their integration into important coding or regulatory regions can have deleterious effects
An evolutional product likely created by such a balance is the programmed DNA elimination in the ciliated protist Tetrahymena, in which the transposon-related sequences are eliminated by a domesticated piggyBac transposase [4]
An evolutional product likely created by such a balance is the programmed DNA elimination in the ciliated protist Tetrahymena, in which the transposon-related sequences are eliminated by the domesticated piggyBac transposase Tpb2p

Summary

Introduction

Transposons represent harmful genetic elements because they potentially rearrange their host’s genome, and their integration into important coding or regulatory regions can have deleterious effects. Transposons are considered ‘‘junk’’ DNAs [1], and hosts have evolved genome defense mechanisms to counteract these selfish elements [2]. An evolutional product likely created by such a balance is the programmed DNA elimination in the ciliated protist Tetrahymena, in which the transposon-related sequences are eliminated by a domesticated piggyBac transposase [4]. One of the products is exchanged with the mating partner and afterwards, the two pronuclei fuse to form the zygote (Fig. 1D). The zygotic nucleus divides twice mitotically (Fig. 1E); of the four mitotic products, two become the new Mics, and the other two develop to become the new Macs (Fig. 1F). The parental Mac is degraded at the end of this process, and the progeny resume vegetative growth when nutrients are supplied (Fig. 1G)

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