The maize P gene encodes a Myb-homologous regulator of red phlobaphene pigment biosynthesis in the pericarp, cob and other floral tissues (Grotewold et al., Cell 76:543-553, 1994). The P locus has a unique structure with two 5.2kb direct repeats flanking the P gene coding region (Lechelt et al., Mol Gen Genet 219:225-234, 1989). When the transposon (Ac) inserts into one site between the two direct repeats in the P-ovov-1114 allele, homologous recombination between the two 5.2kb repeats can occur and the whole P gene coding sequence is deleted (Athma and Peterson, Genetics 128:163-173, 1991).
To further study this transposon-induced homologous recombination, we examined six alleles which carry Ac insertions at different sites in the P locus, in both orientations. Each allele was tested as a heterozygote with P-wr, in the same hybrid (4Co63/W23) genetic background. After the cross with the r-m3::Ds reporter, the mature ears were examined for the presence of colorless pericarp sectors. In a previous study, 80-90% of the colorless pericarp sectors were caused by deletions generated by recombination (Athma and Peterson, Genetics, 128:163-173, 1991). We found the three alleles with Ac inserted at different sites between the two direct repeats had twice the frequency of colorless sectors (average=4.1%) as did three alleles with Ac insertions either within or outside the 5' direct repeat (average=2.0%) (Figure 1). These results suggest that the transposon-induced homologous recombination is enhanced by the insertion of the element between the repeats. Ac orientation had no detectable effect.
To test if transposon-induced recombination is a general phenomenon in plants, we transformed Arabidopsis with a construct (GU.DS.US) containing a stable transposon (Ds) inserted between two deleted GUS parts with homologous direct repeats similar to Ac inserted in maize P locus. Arabidopsis transformants containing GU.DS.US will be crossed with an Arabidopsis strain containing stable Ac, which provides transposase to activate Ds. If transposon-induced homologous recombination occurs between the two deleted GUS parts, the GUS gene will be restored and result in blue sectors after staining with X-gluc reagent.
Figure 1. Frequency of kernels with colorless pericarp sectors observed with Ac inserted at different sites in the P locus.
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