HIGH-LEVEL EXPRESSION OF THE ACTIVATOR TRANSPOSASE GENE INHIBITS THE EXCISION OF DISSOCIATION IN TOBACCO COTYLEDONS

A fusion of the strong cauliflower mosaic virus 35S promoter to the Ac tivator (Ac) transposase (TPASE) gene does not trigger excision of Dis sociation (Ds) continuously during tobacco cotyledon development, alth ough once activated, the 35S promoter remains active throughout embryo geny. Epistasis studies where 35S:TPASE is in trans with later-acting fusions indicate that transient effectiveness for excision results fro m this fusion inhibiting its own action and that of other TPASE source s. Inhibition depends on the strength of TPASE expression, since fusio ns of the 35S promoter to a TPASE cDNA accumulate 30-fold lower amount s of TPASE mRNA than the 35S:TPASE gene fusion and do not inhibit exci sion. We discuss the role of TPASE levels in the curious relationship between Ac activity and Ac dosage in maize.