Transposon Biology

The maize transposable element Activator (Ac) has been introduced into a variety of heterologous plant hosts lacking well-characterized transposable elements with the aim of developing effective gene tagging systems in those species. Ac has been found to be highly active in a number of species including tobacco and tomato, but shows very much reduced levels of activity in Arabidopsis. The focus of our research has been to use a molecular genetic approach to elucidate the basis for the observed low levels of transposon activity in Arabidopsis. A mutagenesis experiment was undertaken with the aim of identifying host factors responsible for repressing the mobility of Ac (Jarvis et al., 1997, Plant J. 11: 907-919). Seed from a line carrying a single copy of the Ac element inserted into the streptomycin phosphotransferase (SPT) reporter fusion, and which displayed typically low levels of Ac activity (the iae progenitor line), were mutagenized using gamma rays. Nineteen mutants displaying high levels of somatic Ac activity were isolated after screening the M2 generation on streptomycin-containing medium (somatic excision of Ac from the SPT::Ac reporter results in green, antibiotic-resistant sectors; see Figure 5). The mutations were shown to fall into two complementation groups (iae1 and iae2, for increased Ac excision), both loci being unlinked to the SPT::Ac insertion. All of the iae mutations segregate in Mendelian fashion, iae1 mutations being recessive and the single iae2 allele being semi-dominant. The iae1-1 and iae2 mutants show 550- and 70-fold increases, respectively, in the average number of Ac excision sectors per cotyledon. Genetic studies also demonstrated that the effect of the iae1-1 mutation on Ac activity was not restricted to the original SPT::Ac insertion site. The iae mutants thus represent a possible means of improving the efficiency of Ac/Ds transposon tagging systems in Arabidopsis, although data suggest that further optimization will be required: molecular analyses of the iae1-1 mutant confirmed the very high levels of Ac excision predicted using the phenotypic SPT excision assay, but revealed only moderate increases in the number of new Ac insertions.

Figure 5: Variegation Phenotype of the iae1 Mutant