Last year we reported the isolation and preliminary characterization of an allele termed P-OVOV (orange variegated pericarp and cob) derived as a change in state of P-VV (MNL 62:42). P-VV specifies colorless pericarp with red sectors, whereas P-OVOV specifies orange pericarp with many dark red sectors, and some colorless sectors. Southern analysis showed that the Ac transposable element is in the opposite orientation in P-OVOV relative to P-VV.
Cloning and sequencing show that inversion of Ac occurred by short range transposition and reinsertion in an inverted orientation. The Ac element has transposed 160 base pairs towards the 5' end of the P locus. Although the Ac element in P-VV is not bordered by host direct repeats, the Ac element in the P-OVOV allele is flanked by 8bp direct repeats of a sequence which is present once in the progenitor P-VV allele.
The orange variegated pericarp phenotype specified by P-OVOV may be considered as a mosaic of three phenotypes:
1. The orange background color may be due to a dilution of the red phlobaphene pigments, resulting from a reduced level of expression of P. Although we do not yet know why P-OVOV allows a moderate level of P expression, an orientation-dependent splicing mechanism similar to that proposed by Wessler, Baran and Varagona (Science 237:916) seems plausible. That is, in P-VV transcripts from P terminate within the Ac element, while in P-OVOV the orientation of Ac may allow splicing out of element sequences during RNA processing.
2. The numerous red sectors may be due to excision of Ac from P-OVOV, thereby restoring a P-RR allele. We know that this is the case for one germinal P-RR revertant from P-OVOV.
3. The occasional light sectors may result from a variety of mutations at P, including deletions (see below) and short-range transpositions. We have characterized two germinal P-VV* mutants which have variegated pericarp and cob resembling the progenitor P-VV allele. Both cases were derived from kernels with variegated pericarp on otherwise orange variegated ears. In one case (P-VV*-4177) Ac has transposed from the site in P-OVOV and inserted at a site approximately 700bp towards the 3' end of the P locus, in the opposite orientation as in P-OVOV (i.e., in the same orientation as the P-VV "grandparent" allele). In the second case (P-VV*-4189) Ac has transposed from the site in P-OVOV and inserted at a site approximately 4kb towards the 5' end of the P locus. The orientation of Ac in P-VV*-4189 is not yet known. In these experiments, we have not detected a strict polarity of transposition as might be predicted from Greenblatt's results showing a 4 map unit region proximal to P which contained no Ac insertions following transposition from P-VV (Greenblatt, Genetics 108:471, 1984). Rather, our results indicate that Ac can transpose in either direction from the site in P-OVOV to other sites within the P locus.
We thank Susan Allan for technical assistance.
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