Regulation of maize Ac/Ds transposition by replication and DNA methylation

In maize the transposable elements Activator/Dissociation (Ac/Ds)
transpose shortly after replication from one of the two resulting
chromatids ("chromatid selectivity"). A model was suggested that
explains this phenomenon as a consequence of different Ac
transposase binding to holo-, hemi- and unmethylated transposon
ends (Wang et al., 1996). It assumes that before replication the
element is holomethylated and does not transpose because TPase can
not bind to the transposon ends. Shortly after replication one of
the two differentially hemimethylated daughter transposons should
become transposition competent. However, DNA methylation-mediated
replication dependence does not completely explain the behavior of
Ac/Ds transposition. In several studies performed in monocot hosts,
it was found that in a transient assay Ac/Ds element excision from
extrachromosomal geminivirus vectors occurs only during vector
replication, although the transfected DNAs were not Cmethylated. In
this work the correlation between Ds transposition, DNA replication
and DNA methylation in the dicot species Petunia hybrida was
studied. Ds reporter vectors harboring TYLCV (Tomato Yellow Leaf
Curl Virus) geminivirus replicon sequences and replicating in
transfected petunia cells were constructed. It has been shown that
the transposition of a Ds element from an extrachromosomal vector
in petunia cells is regulated by DNA replication in a
methylation-dependent and -independent mode. Holomethylation
completely inhibits Ds excision from a non-replicating plasmid,
whereas Ds transposition is restored by replication. Moreover, Ds
elements that are hemimethylated on one DNA strand transpose in the
absence of replication, whereas methylation on the complementary
DNA strand results in at least 6.3-fold reduced excision
frequencies. These data strongly support the transposition model of
Wang et al. (1996). Beyond that, Ds transposition is strongly
promoted by replication also in the absence of methylation. It has
been shown that in petunia cells, unlike monocot hosts, replication
is not a prerequisite for Ds transposition, nevertheless it
enhances Ds transposition by at least a factor of 7.5. Moreover,
replication promotes the formation of a predominant excision
footprint. Implications on the mechanism and regulation of Ac/Ds
transposition are discussed.