Maize Transposable Elements - Part II
Klug and Cummings state (pg 418) "Several Ac and
Ds elements have now been isolated and carefully analyzed.
As a result of this information, the relationship between
the two elements has been clarified. The first Ac element
sequence is 4563 bases long and strikingly similar to
one of the known bacterial transposons. This sequence
contains two 11-base-pair imperfect terminal-inverted
repeats, two open reading frames (ORFs), and three noncoding
regions. Open reading frames contain initiation and
termination sequences and are considered to encode genetic
products. The first Ds element studied (Ds-a) is nearly
identical in structure to Ac except for a 194 base segment
that has been deleted from the largest open reading
frame. There is some evidence that this gene encodes
a transposase enzyme, essential to transposition of
both Ac and Ds elements. The deletion of part of this
gene in a Ds-a element explains its dependence on the
Ac element for transposition. Several other Ds elements
have also been sequenced, and each reveals an even larger
deletion in the same region. In each case, however,
the terminal repeats are retained and seem to be essential
for transposition, provided that a functional transposase
enzyme is suplied by the gene in the Ac element."
Klug and Cummings show in Fig 14.23 that there are two different ways
that the Ds element can change the expression of an allele. One way
is for the Ds element to cause dissociation or chromosome breakage.
When Ds causes a chromosome arm to break, the alleles that are on the
distal end are no longer expressed. This breakage requires the presence
of the Ac element as well as the Ds element. The second way that the
Ds element can change the expression of a gene is for the Ds element
to be inserted within a region of the locus. The RNA and presumably
the protein encoded by the structural gene would then be altered. The
Ds element may later transpose to a different site in the genome which
will restore the wild type of allele product.
