A P-vv allele (P-vv*-5145) specifying extremely light variegated pericarp and cob glumes was obtained from the standard variegated P-vv allele. In crosses to the Ds tester stock r-m3, P-vv*-5145 behaves as though it contains a single Ac element with a normal ability to induce Ds transposition. Southern blot analysis showed that the Ac element in P-vv*-5145 is in the same position and orientation as in the progenitor P-vv allele.
We used PCR to amplify and sequence the termini of the Ac element in the P-vv*-5145 allele. The Ac 5' end was identical to the progenitor P-vv Ac. However, the 3' sequence showed a deletion of two nucleotides: one nucleotide deleted from the 3' end of Ac, and one nucleotide deleted from the flanking P gene sequence:
5' TIR 3' TIR P gene (intron 2)
Progenitor P-vv: CAGGGATGAAA. . . Ac. . .TTTCATCCCTA CCCGTTCGTTTTCGTT
P-vv*-5145: CAGGGATGAAA. . . Ac. . .TTTCATCCCT CCGTTCGTTTTCGTT
Ds (sh), Ds1 (Adh1) TAGGGATGAAA. . . Ds. . .TTTCATCCCTA
The deletion in P-vv*-5145 has the effect of substituting a C for the terminal A in the 3' terminal inverted repeat of Ac. The terminal inverted repeat is critical for transposition, because a deletion of the last four nucleotides (CCTA) of the Ac 3' TIR abolishes transposition (Hehl and Baker, Mol. Gen. Genet. 217:53-59, 1989). However, Ds elements commonly have perfect 11 nucleotide terminal inverted repeats, while the Ac inverted repeat is only 10 nucleotides in length due to mismatch of the terminal nucleotides (see above). Our results demonstrate that, even though the 5' terminal nucleotide can differ between transposition-competent Ac and Ds elements, alteration of the 3' terminal nucleotide can have profound effects on Ac transposition.
The frequency of transposition of Ac in P-vv*-5145 is not known, since germinal revertants haven't been found yet. The frequency of red revertant pericarp sectors is reduced by at least ten fold in P-vv*-5145 relative to the progenitor P-vv. Because the Ac in P-vv*-5145 still transactivates Ds elements normally, P-vv*-5145 may be useful as an Ac source for experiments such as gene tagging with Ds elements and Ds breakage studies in which Ac transposition would lead to undesirable changes in Ac dosage.
I thank Michael Persans, M and Mary Ellen Okar, ME for assistance.
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