MODIFICATION OF THE 5' UNTRANSLATED LEADER REGION OF THE MAIZE ACTIVATOR ELEMENT LEADS TO INCREASED ACTIVITY IN ARABIDOPSIS

In contrast to its behavior in tobacco and tomato, the maize transposa ble element Ac is relatively inactive in Arabidopsis. We show here tha t removal of 537 bp within a CpG-rich region of the Ac 5' untranslated leader region significantly increases the excision frequency of the e lement in Arabidopsis. This increase did not appear to be correlated w ith the removal of sequences that are methylated in inactive Ac elemen ts in maize, as these sites were not methylated in Ac elements in Arab idopsis transformants. The deletion within the 5' untranslated leader did not increase Ac activity by increasing levels of steady-state tran sposase mRNA, as assayed by RNase protection experiments. Moreover, th ere was no correlation between the levels of steady-state transposase mRNA and Ac element activity. This suggests that post-transcriptional regulation of Ac activity occurs in Arabidopsis.