A host factor supports retrotransposition of the TRE5-A population in Dictyostelium cells by suppressing an Argonaute protein.
BACKGROUND
In the compact and haploid genome of Dictyostelium discoideum control of transposon activity is of particular importance to maintain viability. The non-long terminal repeat retrotransposon TRE5-A amplifies continuously in D. discoideum cells even though it produces considerable amounts of minus-strand (antisense) RNA in the presence of an active RNA interference machinery. Removal of the host-encoded C-module-binding factor (CbfA) from D. discoideum cells resulted in a more than 90 % reduction of both plus- and minus-strand RNA of TRE5-A and a strong decrease of the retrotransposition activity of the cellular TRE5-A population. Transcriptome analysis revealed an approximately 230-fold overexpression of the gene coding for the Argonaute-like protein AgnC in a CbfA-depleted mutant.


RESULTS
The D. discoideum genome contains orthologs of RNA-dependent RNA polymerases, Dicer-like proteins, and Argonaute proteins that are supposed to represent RNA interference pathways. We analyzed available mutants in these genes for altered expression of TRE5-A. We found that the retrotransposon was overexpressed in mutants lacking the Argonaute proteins AgnC and AgnE. Because the agnC gene is barely expressed in wild-type cells, probably due to repression by CbfA, we employed a new method of promoter-swapping to overexpress agnC in a CbfA-independent manner. In these strains we established an in vivo retrotransposition assay that determines the retrotransposition frequency of the cellular TRE5-A population. We observed that both the TRE5-A steady-state RNA level and retrotransposition rate dropped to less than 10 % of wild-type in the agnC overexpressor strains.


CONCLUSIONS
The data suggest that TRE5-A amplification is controlled by a distinct pathway of the Dictyostelium RNA interference machinery that does not require RNA-dependent RNA polymerases but involves AgnC. This control is at least partially overcome by the activity of CbfA, a factor derived from the retrotransposon's host. This unusual regulation of mobile element activity most likely had a profound effect on genome evolution in D. discoideum.
Department of Pharmaceutical Biology, Institute of Pharmacy, University of Jena, Semmelweisstrasse 10, 07743 Jena, Germany.
Anika
A
Schmith
Department of Pharmaceutical Biology, Institute of Pharmacy, University of Jena, Semmelweisstrasse 10, 07743 Jena, Germany.
Thomas
T
Spaller
Department of Pharmaceutical Biology, Institute of Pharmacy, University of Jena, Semmelweisstrasse 10, 07743 Jena, Germany.
Friedemann
F
Gaube
Department of Molecular Biology, Biomedical Center, Swedish University of Agricultural Sciences, Uppsala, Sweden ; Present address: Aprea AB, Karolinska Institutet Science Park, Nobels väg 3, 17175 Solna, Sweden.
Åsa
Å
Fransson
Institute of Biology - Genetics, University of Kassel, Kassel, Germany.
Benjamin
B
Boesler
Ribogenetics@Biochemistry Lab, Department of Life Sciences and Chemistry, Molecular Life Sciences Research Center, Jacobs University Bremen, Bremen, Germany.
Sandeep
S
Ojha
Institute of Biology - Genetics, University of Kassel, Kassel, Germany ; Present address: Department of Biology, Brawijaya University, Jl. Veteran, Malang, East Java Indonesia.
Wolfgang
W
Nellen
Ribogenetics@Biochemistry Lab, Department of Life Sciences and Chemistry, Molecular Life Sciences Research Center, Jacobs University Bremen, Bremen, Germany.
Christian
C
Hammann
Department of Cell and Molecular Biology, Biomedical Center, Uppsala University, Uppsala, Sweden.
Fredrik
F
Söderbom
Department of Pharmaceutical Biology, Institute of Pharmacy, University of Jena, Semmelweisstrasse 10, 07743 Jena, Germany.
Thomas
T
Winckler
Argonaute; Dictyostelium; RNAi; Retrotransposition; siRNA