A bacterial streptomycin resistance gene (SPT) was engineered to make it possible to detect visually the transposition of the maize transposon Activator (Ac) in tobacco. In the presence of streptomycin, transgenic seedlings carrying the SPT gene appear green, whereas those carrying an SPT:: Ac construct display clones of green cells on a white background. Fully green seedlings arise in the progeny of SPT:: Ac transformants as a result of excision of Ac before fertilization. About half of these germinal revertants carry a transposed Ac element. Therefore, SPT:: Ac constitutes an efficient marker for selecting plants that have undergone transposition. In maize, there is a negative effect of increasing Ac dosage on the frequency and timing of Ac transposition. This negative effect is not observed in tobacco with the streptomycin variegation assay.
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