THE AC-ST2 ELEMENT OF MAIZE EXHIBITS A POSITIVE DOSAGE EFFECT AND EPIGENETIC REGULATION

A novel derivative of the maize transposable element Ac, termed Ac-st2 , that displays a positive dosage effect in maize has been identified. Although identical in sequence to other Ac elements, increasing the c opy number of the element in the endosperm results in earlier and more frequent Ds excisions. Ac-st2 autonomously transposes and catalyzes s omatic excisions of Ds elements. Germinal transpositions of either Ac- st2 or Ds, however, were not observed. The Ac-st2 phenotype includes a reduction in Ac transcript accumulation that is associated with incre ased methylation at specific sites in the promoter region of the major transcriptional start site within Ac (ORFa). This element differs fro m metastable (cycling) Ac derivatives in that Ac-st2 conditions a unif orm transposition pattern throughout endosperm and plant development. Ac-st2 undergoes frequent increases in activity after its association with an active Ac element This change in activity correlates with redu ced levels of methylation in the ORFa promoter region. Using a competi tive PCR assay, Ac transcript accumulation was followed through endosp erm develop ment. From these data, a model is proposed to explain the patterns of variegation associated with both ''wild type'' active Ac a nd Ac-st2 elements.