The frequency of transposition of the maize element Activator is not affected by an adjacent deletion

Summary

The maize mutable allele bz-m2 (Ac), which arose from insertion of the 4.6 kb Ac element in the bz (bronze) locus, gives rise to stable bz (bz-s) derivatives that retain an active Ac element closely linked to bz. In the derivative bz-s:2114 (Ac), the Ac element is recombinationally inseparable from bz and transposes to unlinked sites at a frequency similar to that in the progenitor allele bzm2 (Ac). Both alleles have been cloned and sequenced. The bz-s:2114 (Ac) mutation retains Ac at the original site of insertion, but has lost a 789 pb upstream bz sequence adjacent to the insertion, hence the stable phenotype. The 8 bp target site direct repeat flanking the Ac insertion in the bz-m2 (Ac) allele is deleted in bz-s: 2114 (Ac), yet the Ac element is not impaired in its ability to transpose. The only functional Ac element in bz-s:2114 (Ac) is the one at the bz locus: in second-cycle derivatives without Ac activity, the loss of Ac activity correlated with the physical loss of the Ac element from the bz locus. The deletion endpoint in bz-s: 2114 (Ac) corresponds exactly with the site of insertion of a Ds element in a different bz mutation, which suggests that there may be preferred integration sites in the genome and that the deletion originated as the consequence of an abortive transposition event. Finally, we report two errors in the published Ac sequence.