Ds elements, unable to excise themselves, can be excised and moved to a new location if an Ac element is present somewhere in the genome. Thus an Ac (activator) element activates transposition of a Ds element. In the absence of an Ac element, a Ds element remains in the gene and the allele behaves like a recessive allele. The Ac element is an autonomous transposon because of its capability of self-transposition. the Ds element is called a nonautonomous transposon as it is incapable of self-transposition. However, the Ds element can be transposed if an autonomous transposon is present in the genome.
Ac and Ds elements transpose in somatic cells only; in germ line cells they transpose rarely. Several Ac and Ds elements have been cloned and sequenced by Nina Fedoroff and her colleagues. The nucleotide sequences of these elements have revealed two distinct features about the Ac element and other autonomous elements:
1) Each one is bordered by “imperfect inverted terminal repeats”.
2) Each element carries a gene encoding the transposase enzyme.
Ds elements, however, contain deletions which include part of the transposase gene. Thus each Ds element is unable to encode functional trasnposase; and therefore could not transpose independently.