ALTERNATIVE PROCESSING OF THE MAIZE AC TRANSCRIPT IN ARABIDOPSIS

The successful application of the maize transposable element system Ac /Ds as a genome mutagen in heterologous plant species has recently pro ved the versatility and power of this technique in plant molecular bio logy. However, the frequency of Ac/Ds transposition is considerably lo wer in Arabidopsis thaliana than in most other dicot plant species tha t have been studied. Since previous research has established that tran scripts derived from monocot genes can be alternatively processed in d icot plants, we have investigated both the efficiency of intron splici ng and polyadenylation of the maize Ac transposase pre-mRNA in Arabido psis thaliana, Nicotiana tabacum, Nicotiana plumbaginifolia and Zea ma ys. In this paper, we demonstrate that intron 4 is alternatively splic ed within Arabidopsis, using cryptic 5' and 3' splice sites within the intron sequence, leading to a heterogeneous population of full length transposase transcript. Furthermore, analysis of transposase transcri pt polyadenylation revealed that at least four alternative poly(A) sit es were utilized between introns 2 and 3, resulting in truncated trans posase transcripts. Finally, by Northern blotting, we established that the truncated transposase transcript was the most abundant form of tr ansposase message in Arabidopsis. In contrast to these findings, the a lternative splicing and premature polyadenylation of Ac message in Ara bidopsis was unparalleled in the other species examined. We suggest th at the poor frequency of transposition of Ac in Arabidopsis may be in part due to the low quantity of correctly processed transposase transc ript available in this species.