Effects of gene dosage and sequence modification on the frequency and timing of transposition of the maize element Activator (Ac) in tobacco

The effect of Ac copy number on the frequency and timing of germinal transposition in tobacco was investigated using the streptomycin phosphotransferase gene (SPT) as an excision marker. The activity of one and two copies of the element was compared by selecting heterozygous and homozygous progeny of transformants carrying single SPT::Ac inserts. It was observed that increasing gene copy not only increases the transposition frequency, but also occasionally alters the timing of transposition such that earlier events are obtained. The result is that some homozygous plants generate multiple streptomycin resistant progeny carrying the same transposed Ac (trAc) element. We have also investigated the effect of modification of the sequence in the region around 82 bp downstream of the polyadenylation site and 177 bp from the 3 end of the element on germinal excision frequencies. Alteration of three bases to create a BglII site at this location caused a minor decrease in germinal excision events, but insertion of four bases to create a Cla I site caused a 10-fold decrease in the transposition activity of the Ac element.