Variations in the maize Ac transposase transcript level and the Ds excision frequency in transgenic wheat callus...

To investigate the excision of a maize transposable element in wheat cells, plasmid DNAs containing a Dissociation (Ds) element located between a rice actin 1 gene promoter and a beta-glucuronidase (GUS) gene (gus) were introduced into wheat callus lines by microprojectile bombardment, and transient GUS expression was assayed. The gus-expressing cells after Ds excision were detected only when the Activator (Ac) transposase gene was co-transformed. To further examine a relationship between the amount of Ac mRNA and the Ds excision frequency, the Ds-containing plasmids were introduced into 15 independent transgenic callus lines transformed with the Ac transposase gene. Ten lines expressed the Ac transposase gene under the control of either the cauliflower mosaic virus 35S promoter or the Ac native promoter. The gus gene expression that indicated the Ds excision was observed only in the transgenic callus lines stably expressing the Ac transposase gene. The number of blue spots reflecting the frequency of Ds excision was variable among them. Northern-blot analysis also showed a large variability in the amount of Ac transposase transcripts among the lines. It was however noted that the excision frequency was decreased at a high level of the Ac transposase transcripts, supporting the hypothesis that Ds excision is inhibited above a certain level of the Ac transposase as observed in maize and transgenic tobacco.