Targeted transposition is the replacement of one P element with anothe
r. We are exploiting this unique property of P elements to study the c
omplex regulatory domain of the Dopa decarboxylase (Ddc) gene in Droso
phila melanogaster. P element constructs targeted to the same site in
the genome will be subjected to the same position effect. This allows
the subtle effects typical of most mutations in the Ddc regulatory reg
ion to be measured in the absence of the variable influences of positi
on effects which are associated with the current method of germline tr
ansformation. We have investigated some of the parameters affecting ta
rgeted transposition of a Ddc transposon, P[Ddc], into a P element all
ele at the vestigial locus. These events were detected by an increased
mutant vg phenotype. The location of the donor transposon in cis or i
n trans to the target had little effect on the frequency of targeting.
Likewise, the mobility of different donor elements, as measured by th
eir rate of transposition to a different chromosome, varied nearly 20-
fold, while the rate of targeted transposition was very similar betwee
n them. All targeted alleles were precise replacements of the target P
element by P[Dcd], but in several cases the donor was inserted in the
opposite orientation. The targeted alleles could be described as the
result of a replicative, conversion-like event.