We visualized the transposase (TPase) of the maize transposable element Activator (Ac) by immunofluorescence in maize endosperm and in transfected petunia protoplasts. The TPase is detected in the nuclei of both, where it aggregates into large, rod-like complexes about 2 µm in length. Outside of these aggregates, no TPase is detectable. In petunia protoplasts the amount of the complexes is directly related to the strength of the promoter fused to the Ac coding region. However, no correlation is seen between the excision frequency of a Ds element in the petunia protoplast assay and the amount of TPase aggregates. This is an indication that the TPase protein bound in the aggregates has no TPase activity. We therefore consider the possibility that TPase aggregation serves as a sequestration mechanism which controls TPase activity in the cell. Consistent with such a posttranslational mechanism is the observation that in transgenic tobacco transpositions occur only below a certain threshold in TPase expression (Scofield et al., Cell 75:507-517, 1993).

A functional TPase derivative, lacking the amino-terminal 102 amino acids, differs from the full-length TPase with respect to the formation of aggregates. At low expression levels, no difference in nuclear accumulation is observed between the two proteins. At high expression levels, however, aggregates of the truncated TPase appear in the cytoplasm of many cells, and the amount of nuclear aggregates does not exceed a certain level. In contrast, the wild type TPase still almost exclusively accumulates in the nuclei. Therefore, the N-terminus of the TPase contains sequences involved in nuclear localization or aggregation of the protein. 

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