Ac transposon-induced recombination at the P locus
--Yongli Xiao and Thomas Peterson

The P gene encodes a Myb-homologous transcriptional regulator of flavonoid biosynthesis in the kernel pericarp, cob glumes, and other floral organs (Grotewold et al., Cell 76:543-553). Using the transposable element Ac as a tag, the P gene was cloned and found to have a unique structure: the coding sequence is flanked by two long (5.2 kb) repeats in direct orientation (Lechelt et al., Mol. Gen. Genet. 219:225-234; see Figure 1 in following report by Zhang and Peterson). We subsequently demonstrated that the Ac element, when inserted between the two direct repeats in the P-ovov-1114 allele, induces a significant level of homologous recombination between the repeats (Athma and Peterson, Genetics 128:163-173). A similar enhancement of recombination has been shown to be induced by Mu transposons inserted in direct repeats at the maize Knotted locus (Lowe et al., Genetics 132:813-822).

To further characterize the mechanism of this transposon-induced recombination, we have compared the mutability of six P gene alleles with Ac insertions at different sites in the locus. Each allele was tested as a heterozygote with P-wr, in the same hybrid (4Co63/W23) genetic background. Plants were detasseled and allowed to pollinate with the r-m3::Ds reporter stock, and the mature ears were examined for the presence of colorless pericarp sectors indicative of P gene mutations, including deletions produced by recombination at the P locus. The results can be summarized briefly as follows: the three alleles with Ac inserted at various sites between the direct repeats of the P locus had twice the frequency of pericarp sectors (3.8%) as did three alleles with Ac insertions either within or outside the 5' direct repeat (1.9%). We have previously estimated that, for the P-ovov-1114 allele, 80 to 90% of the colorless pericarp sectors are due to deletions generated by recombination, and 10 to 20% are due to intragenic transpositions and other mutations (Athma and Peterson, 1991). These results suggest that the ability of a transposable element to stimulate recombination between repeated sequences is significantly enhanced by insertion of the element between the repeat sequences. 

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