dictyNews Volume 41 Number 18

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dictyNews 
Electronic Edition 
Volume 41, number 18 
August 28, 2015 

Please submit abstracts of your papers as soon as they have been 
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========= 
Abstracts 
========= 

 
A host factor supports retrotransposition of the TRE5-A population  
in Dictyostelium cells by suppressing an Argonaute protein 

Anika Schmith1, Thomas Spaller1, Friedemann Gaube1, sa Fransson2,  
Benjamin Boesler3, Sandeep Ojha4, Wolfgang Nellen3,  
Christian Hammann4, Fredrik Sšderbom5, Thomas Winckler1 

1 Department of Pharmaceutical Biology, Institute of Pharmacy,  
University of Jena, Germany 
2 Department of Molecular Biology, Biomedical Center, Swedish  
University of Agricultural Sciences, Uppsala, Sweden 
3 Institute of Biology Ð Genetics, University of Kassel, Germany 
4 Ribogenetics@Biochemistry Lab, Department of Life Sciences  
and Chemistry, Molecular Life Sciences Research Center, Jacobs  
University Bremen, Germany 
5 Department of Cell and Molecular Biology, Biomedical Center,  
Uppsala University, Sweden 

 
Mobile DNA, in press 

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. 

 
Submitted by Thomas Winckler [t.winckler@uni-jena.de]  
---------------------------------------------------------------------- 

 
Organization of microtubule assemblies in Dictyostelium syncytia  
depends on the microtubule crosslinker, Ase1 

Irina Tikhonenko, Karen Irizarry, Alexey Khodjakov, and  
Michael P. Koonce 

Division of Translational Medicine 
Wadsworth Center 
NYS Department of Health 
Albany, NY 12201-0509 

 
Cell Molecular Life Sciences, in press 

It has long been known that the interphase microtubule (MT) array  
is a key cellular scaffold that provides structural support and  
directs organelle trafficking in eukaryotic cells. Although in  
animal cells, a combination of centrosome nucleating properties  
and polymer dynamics at the distal microtubule ends is generally  
sufficient to establish a radial, polar array of MTs, little is  
known about how effector proteins (motors and crosslinkers) are  
coordinated to produce the diversity of interphase MT array  
morphologies found in nature. This diversity is particularly  
important in multinucleated environments where multiple MT arrays  
must coexist and function. We initiate here a study to address  
the higher ordered coordination of multiple, independent MT  
arrays in a common cytoplasm. Deletion of a MT crosslinker of the  
MAP65/Ase1/PRC1 family disrupts the spatial integrity of multiple  
arrays in Dictyostelium discoideum, reducing the distance between  
centrosomes and increasing the intermingling of MTs with opposite  
polarity. This result, coupled with previous dynein disruptions  
suggest a robust mechanism by which interphase MT arrays can  
utilize motors and crosslinkers to sense their position and  
minimize overlap in a common cytoplasm. 

 
Submitted by Mike Koonce [michael.koonce@health.ny.gov]  
============================================================== 
[End dictyNews, volume 41, number 18]