Two independent insertions of an Ac element into the waxy locus (McClintock, Carnegie Inst. Wash. Yearb. 62:486, 1963 and 63:592, 1964) have recently been cloned, Ac9 and Ac7 (Fedoroff et al., Cell 35:235, 1983; Behrens et al., MGG 194:346, 1984). The pattern of somatic reversions from the waxy phenotype to normal is different in the two alleles, since in homozygous plants reversion events occur earlier in endosperm development with the wx-m7 than with the wx-m9 allele (O. Nelson, pers. comm.). Furthermore, wx-m7 is not a null mutation. The wx-m7 allele carries Ac near the 5' end of the gene, while the Ac element in the wx-m9 allele is inserted in an exon 2.5 kb downstream from this site (Fedoroff et al., Cell 35:235, 1983; Behrens et al., MGG 194:346, 1984; A. Gierl and Zs. Schwarz-Sommer, pers. comm.). The DNA sequence of Ac7 has been determined (Mueller-Neumann et al., MGG 198:19, 1984) and compared to that of Ac9, which has been sequenced by Pohlman et al. (Cell 37:635, 1984).
In the sequence, three open reading frames (ORF) are detected, two of which read to the left (ORF1 and ORF2), the other to the right (ORF3). Two of the ORFs qualify as potential protein-encoding genes. We have no information about splicing, and RNA and protein studies are needed.
All Ds elements sequenced so far (Doring et al., Nature 307:127, 1984; Pohlman et al., Cell 37:635, 1984; Sutton et al., Science 223:1265, 1984; Merckelbach, pers. comm.) contain an 11 bp perfect inverted repeat at their ends: TAGGGATGAAA. Ac, however, shows a replacement of the 5' terminal T by a C residue at one end. This base substitution does not abolish transposition capability. Variability of the outermost nucleotides has not yet been observed in other transposons.
Ac contains other inverted and even more direct repeats, the latter mostly clustered in the terminal 1 kb sequences. The inverted sequences can partially be used to build up secondary structures. The base composition of Ac is inhomogeneous. ORF3 contains more CpG dinucleotides than expected on a random basis, which is interesting, because these sites are sensitive to methylation.
The DNA sequences of the Ac elements of both the wx-m7 and the wx-m9 alleles are identical. The phenotypic differences between the two alleles must therefore have another cause. It will be interesting to see whether the different insertion sites are responsible for the respective phenotypes, as has been suggested for enhancer elements by Peterson (In: Bukhari, ed., CSH Lab., NY, p. 429, 1977).
M. Muller-Neumann, J. Yoder, P. Starlinger
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