DNA methylation is not necessary for the inactivation of the Tam3 transposon at non-permissive temperature in Antirrhinum

It has been proposed that DNA methylation plays an important role in the inactivation of transposons. This view stems from a comparison of the degree of methylation of transposons in the active and inactive state. However, direct evidence for the degree of methylation required for the suppression of transposition has not been reported. Transposon Tam3 in Antirrhinum majus undergoes somatic reversal of its transposition activity, which is tightly controlled by temperature: low temperature around 15 °C permits transposition, high temperatures around 25 °C strongly inhibits it. Our previous study had shown that the methylation state of the Tam3 end regions is negatively correlated with the Tam3 transposition frequency. The results of the present study reveal that the inactive state of Tam3 copies at high temperature is unlikely to be directly coupled to the methylation state. Treatment with methylation inhibitors (5-azacytidine or 5-azacytidine+ethionine) does not affect Tam3 excision frequency in calli derived from Antirrhinum hypocotyls. The results suggest that methylation is not essential for the suppression of Tam3 transposition at high temperature, but rather that some other mechanism(s) involved in the control of Tam3 transposition may be obscured by methylation.