- Specific binding ofNicotiana nuclear protein(s) to subterminal regions of theAc transposable element was detected using gel mobility shift assays. A sequence motif (GGTAAA) repeated in both terminal regions ofAc, was identified as the protein binding site. Mutation of two nucleotides in this motif was sufficient to abolish binding. Based on a series of competition assays, it is deduced that there is cooperative binding between two repeats, each similar to the GGTAAA motif. The binding protein is probably similar to a previously characterized maize protein which binds to a GGTAAA-containing motif located in the ends ofMutator. Moreover, we show that DNA fromDs1 competes for protein binding toAc termini, and we show, by sequence analysis, that GGTAAA binding sites are present in the terminal region ofTgm1, Tpn1, En/Spm, Tam3 andDs1-like elements. This suggests that the binding protein(s) might be involved in the transposition process.