DNA Information - Structure

The Ac transposable element is 4563 bases long with two 11- base pair imperfect terminal-inverted repeats (TIR), two open reading frames (ORF), and three noncoding regions. It is characteristic of transposable elements to have TIR sequences at each end of the element. Open reading frames are DNA sequences on chromosomes which "contain initiation and termination sequences and are considered to encode genetic products." The ORF's of the Ac transposable element encode a transpose enzyme which is essential to transposition of the transposable element. The Ds element requires the Ac element for activation. The Ds element is deficient in a portion of the transposase gene. This explains why Ds requires the presence of Ac to transpose Ds to a new site in the genome.

from - pg. 418-419 in Concepts in Genetics by Klug and Cummings

"The sequences at the terminals of a transposon are bidirectionally and rotationally symmetrical, that is, the same sequence appears at both ends of the element, but it reads in the opposite direction on opposite strands. The inverted repeats serve as recognition signals demarcating the sequence to be transposed. There are two transposition enzymes, a transposase and a resolvase. The transposase begins the transposisiton process and the resolvase finishes it.... The resolvase itself is the regulator. In addition to acting as an enzyme, the protein acts as a repressor. It binds to a site between the transposase gene and its own gene, preventing the expression of both genes."

from -Fedoroff, N.V. 1984.

Movement "The momement of transposition comes after Mp itself has replicated: but often before the chromosome on which it resides has completed its replication. Only one of the two daughter copies of the element moves, and most often it moves to a nearby site on the same chromosome. If the recipient site has not yet replicated, its subsequent replication gives rise to one daughter chromosome carrying two copies of Mp and one chromosome with a single copy at a new site. If the element moves to a site that has already replicated, one daughter chromosome is without an Mp and the other carries two copies of the element, one of which is at a new site." [The Mp element is another symbol for the Ac element.]

from -Federoff, N.V. 1984.

DNA Footprints "Unlike most procaryotic and some eucaryotic transposable elements, Ds is not excised precisely when the mutant allele reverts to the wild type. Sachs et al. (1983) have sequenced several revertants of the DS1 insertion in the alcohol dehydrogenase gene. They found the 8 bp duplication of host DNA still present after excision of Ds1.... If Ac or Ds insertions in an exon are excised leaving some of the duplicated base pairs behind, this must alter protein structure either by causing a frame shift or by adding a few amino acids to the protein.... The fact that the small duplications of host DNA created upon insertion of the element are retained in a slightly altered form after excision may indicate that plant transposable elements play a role in the evolution of plant genes. Not only can these elements abolish or alter the gene funtion by insertion, but they can also exert a mutator activity when they are excised from their position in the genome. They create a new type of mutation by inserting small blocks of nucleotides at the positions where they have visited the chromosome. If the insertion is in a coding sequence, this might lead to the addition of one, two, or three amino acids, depending of the length of the duplication."

from -Doring, H.P., and P. Starlinger. 1984.

Methylation of transposable elements. The Spm transposon is an 8.4 kb element found in the maize genome. "The primary Spm transcript is alternatively spliced, yielding mRNA sequences with different open reading frames (ORFs), some with extensive overlaps. Four large transcripts have been identified and designated tnpA-D (Masson et al., 1989) Each transcript encodes either just one or one unique ORF, and the tnpA transcript is the shortest and most abundant." ... "Molecular studies showed that there are methylation differences between genetically silent and active Spm elements (Banks et al., 1988). Overall, both active and inactive elements are extensively methylated, but the sequences flanking the transposon at the insertion site are not. Active Spm elements differ from inactive ones by the absence of methylation in a short region of about 0.6 kb surrounding the transcription start site. Active Spm transposons are unmethylated in the 0.2 promoter region and inactive transposons are methylated.... Extensively methylated Spm transposons are not transcriped, do not transpose autonomously and are mobilized at a very low frequency by an active element. This suggests that methylation inhibits both transcription and transposition.... Of the 4 Spm-encoded proteins, only TnpA can reactivate the transposon, identifying it as the epigenetic activator. Reactivation is accompanied by a decrease in 5' terminal methylation of the transposon.... Using domain-swapping experiments, it was found that TnpA functions only as a repressor, although it can be converted to an efficient transcription activation domain (Schlappi et al., 1996).

from -Fedoroff, N.V. 1999.