Molecular basis on the regulation of methylation that controls transposition of the plant transposons

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The transposition frequency of Tam3 in Antirrhinum, unlike that of most other cut-and-paste type transposons, is tightly controlled by temperature : Tam3 transposes rarely at 25℃, but much more frequently at 15℃. Here we studied the mechanism of the low-temperature-dependent transposition (LTDT) of Tam3. Our results strongly suggest that LTDT is not likely to be due to either transcriptional regulation or posttranscriptional regulation of the Tam3 Tpase gene. We found that temperature shift induced a remarkable change of the methylation state unique to Tam3 sequences in the genome : higher temperature resulted in hypermethylation, while lower temperature resulted in reduced methylation. The methylation state was reversible within a single generation in response to a temperature shift. Although our data demonstrate a close link between LTDT and the methylation of Tam3, they also suggest that secondary factor(s) other than DNA methylation is involved in repression of Tam3 transposition.