Studies of the Ac (Activator) transposable element provided the data which led Barbara McClintock to postulate that certain segments of chromosomes could transpose to different locations in the genome. McClintock also recognized the existence of Ds (Dissociation) elements which could transpose, but only in the presence of a trans-acting Ac element elsewhere in the genome. DNA sequences corresponding to Ds and Ac have now been identified, and an understanding of many of the properties of these transposable elements in the maize genome has been acquired in recent years.

It is known that cryptic Ac elements and members of at least two families of Ds elements occur in the genome of all maize lines examined. Ds elements also occur in Teosinte and the more distantly related Tripsacum. We discuss the possible origin of these elements and consider the mechanism of activation of cryptic Ac elements. A recent molecular analysis of a transition of an Ac-derived Ds-element back to an active Ac element suggests one molecular mechanism by which changes in the activity state of Ac may occur.

Distinctive phenotypes created by controlling elements within a target gene have been shown to be governed by the properties of the insertion element and the position of the insertion within the gene. Genetic effects include modulation of gene expression, alteration of gene products, instability of mutant phenotypes, deletion and duplication of chromosome segments and the production of chromosome rearrangements. We describe an example where a Ds insertion generates an additional intron in the Adh1 gene which reduces gene expression through mRNA instability. We also discuss an Ac-dependent modulation of P gene activity in glume and pericarp tissues of maize which may be attributed to an alteration either in patterns of gene expression or the developmental biology of the flower.

The molecular consequences of Ac and Ds insertions and excisions are known at the DNA sequence level but little is known of the mechanism of transposition. An initial approach has been to analyse Ac transcription. Preliminary results showing transcription of a limited region of Ac are discussed. The corresponding upstream regions have been linked to the coding region of chloramphenicol acetyltransferase (CAT) and show promoter activity following electroporation into tobacco protoplasts.

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