The maize transposable element Activator (Ac) encodes a transposase (TPase) protein, whose DNAbinding domain is located in a basic region around aa 200. The N-terminal 102 aa of the TPase are not required for the transposition reaction. In transfected petunia protoplasts, we analyzed the protein levels of the N-terminafly truncated TPase and mutants thereof and the corresponding transposition frequencies. The TPase protein forms large insoluble aggregates at high expression levels. There is no proportionality observed between TPase levels and transposition frequency. Twenty-one mutations (of 26), which are distributed over 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 inhibition of the active TPase is not caused by competition for the binding sites on the transposon. Accordingly, Ac TPase acts as an oligo-or multimer formed by protein-protein interactions. Peculiarly, two mutants lacking 53 and 98 aa from the C terminus that are themselves transpositionally inactive lead to an increased excision frequency when they are coexpressed with the active truncated TPase.

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A cDNA clone for maize endosperm sucrose synthetase of 62o nucleotide pairs length was obtained by cloning double stranded DNA obtained from the total maize endosperm poly(A)RNA in pBR322, and identifying the appropriate clone by hybrid-promoted translation.In Southern blotting to genomic BamHI-digested DNA, a single band only of-v2o Kb lights up, indicating that the sucrose syn-

mRNA of 23‐day‐old maize endosperm was translated both in wheat germ extracts and rabbit reticulocyte lysates. A protein with an apparent molecular weight of 88000 comigrates in dodecylsulfate/polyacrylamide electrophoresis with sucrose synthase. This protein is precipitated with an antiserum against sucrose synthase and shows the same protease digestion pattern as the enzyme. It is not synthesized with mRNA extracted from sh/sh mutant kernels lacking sucrose synthase. By these criteria, the protein is the translation product in vitro of sucrose synthase mRNA. The separation of mRNA in methylmercury‐hydroxide‐agarose gels and subsequent translation indicates a length of sucrose synthase mRNA of 2800 nucleotides which is compatible with the coding length necessary for a protein with a molecular weight of 88000 plus untranslated sequences.