Abstract
BackgroundTransposable elements (TEs) are common constituents of centromeres. However, it is not known what causes this relationship. Schizosaccharomyces japonicus contains 10 families of Long Terminal Repeat (LTR)-retrotransposons and these elements cluster in centromeres and telomeres. In the related yeast, Schizosaccharomyces pombe LTR-retrotransposons Tf1 and Tf2 are distributed in the promoter regions of RNA pol II transcribed genes. Sequence analysis of TEs indicates that Tj1 of S. japonicus is related to Tf1 and Tf2, and uses the same mechanism of self-primed reverse transcription. Thus, we wondered why these related retrotransposons localized in different regions of the genome.ResultsTo characterize the integration behavior of Tj1 we expressed it in S. pombe. We found Tj1 was active and capable of generating de novo integration in the chromosomes of S. pombe. The expression of Tj1 is similar to Type C retroviruses in that a stop codon at the end of Gag must be present for efficient integration. 17 inserts were sequenced, 13 occurred within 12 bp upstream of tRNA genes and 3 occurred at other RNA pol III transcribed genes. The link between Tj1 integration and RNA pol III transcription is reminiscent of Ty3, an LTR-retrotransposon of Saccharomyces cerevisiae that interacts with TFIIIB and integrates upstream of tRNA genes.ConclusionThe integration of Tj1 upstream of tRNA genes and the centromeric clustering of tRNA genes in S. japonicus demonstrate that the clustering of this TE in centromere sequences is due to a unique pattern of integration.
Highlights
Transposable elements (TEs) are common constituents of centromeres
To detect transposition events S. pombe patches expressing the three versions of Tj1 marked with neo (Tj1-neo) were replica printed to medium containing 5-fluoroorotic acid (5FOA)
In the final step of the assay the patches were replica printed to media containing 5FOA and G418 to test for the presence of integrated copies of Tj1-neo (Fig. 2c)
Summary
Transposable elements (TEs) are common constituents of centromeres It is not known what causes this relationship. Schizosaccharomyces japonicus contains 10 families of Long Terminal Repeat (LTR)-retrotransposons and these elements cluster in centromeres and telomeres. Whole genome sequencing of the fission yeast Schizosaccharomyces japonicus showed large numbers of transposable elements in the centromeres of all three chromosomes (Fig. 1a) [6]. Analysis of the S. japonicus genome identified 13 full length long terminal repeat (LTR)-retrotransposons and 251 fragments of LTRretrotransposons that together comprise 309 kb or 2.6 % of the 11.7 Mb genome of S. japonicus [6]. Phylogenetic analysis revealed that these transposable element sequences are derived from 10 distinct families of LTRretrotransposons named Tj1 through Tj10 that are all members of the gypsy/Ty3 group of LTR-retrotransposons.
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