DOMINANT TRANSPOSITION-DEFICIENT MUTANTS OF MAIZE ACTIVATOR (AC) TRANSPOSASE

The maize transposable element Activator (Ac) encodes a transposase (T Pase) protein, whose DNA-binding domain is located in a basic region a round aa 200. The N-terminal 102 aa of the TPase are not required for the transposition reaction. In transfected petunia protoplasts, we ana lyzed the protein levels of the N-terminally truncated TPase and mutan ts thereof and the corresponding transposition frequencies. The TPase protein forms large insoluble aggregates at high expression levels. Th ere is no proportionality observed between TPase levels and transposit ion frequency. Twenty-one mutations (of 26), which are distributed ove r the whole length of the protein, inactivate the TPase completely. By coexpressing inactive mutant and active truncated TPase, it was found that several mutations have a trans-dominant inhibitory effect. Among those are two DNA-binding-deficient mutants, indicating that inhibiti on of the active TPase is not caused by competition for the binding si tes on the transposon. Accordingly, Ac TPase acts as an oligo- or mult imer formed by protein-protein interactions. Peculiarly, two mutants l acking 53 and 98 aa from the C terminus that are themselves transposit ionally inactive lead to an increased excision frequency when they are coexpressed with the active truncated TPase.