The presence of enhancers adjacent to the Ac promoter increases the abundance of transposase mRNA and alters the timing of Ds excision in Arabidopsis.

Two copies of domain B of the CaMV 35S promoter were inserted ca. 300 bp upstream of the transcriptional start site of the Ac transposase gene. Four independent Arabidopsis transformants containing this fusion (35SenhAc::TPase) were made and the abundance of transposase mRNA in each of them was determined. The presence of the enhancers increased the abundance of the transposase mRNA by about 12-fold compared to that found in plants containing an Ac promoter fusion to the transposase gene (Ac::TPase). Hybrid plants carrying 35SenhAc::TPase and a Ds element inserted in a streptomycin phosphotransferase (SPT) gene were constructed and the frequency with which Ds excision occurred in the developing cotyledons was measured. Moreover, the number of progeny of these hybrid plants which inherited an SPT gene activated by Ds excision was studied in individual F2 families. Those derived from 35SenhAc::TPase often contained higher proportions of streptomycin-resistant (strepR) F2 progeny than those derived from Ac::TPase. These high frequencies of strepR seedlings were comparable to those previously detected after activation of Ds by a CaMV 35S promoter fusion to transposase (35S::TPase), but occurred in fewer families. The higher frequency with which this occurred in families derived from 35SenhAc::TPase compared to Ac::TPase suggests that the presence of enhancers adjacent to the native Ac promoter can influence transposase gene expression, and in this case often results in earlier excision of Ds during plant development.