Ac sequences act autonomously in that they can control their own transpositions. They work in trans and probably produce a transposase, although the Ac product has not yet been isolated nor identified. Ds is a transposable element which behaves as though it lacks transposase activity and moves only in the presence of an Ac element. Thus, an alteration in the transposase portion of the Ac element should be sufficient to convert Ac to Ds, and such conversions have been described by McClintock. Molecular analysis of wx-m9Ds, a Ds derivative of Ac in the waxy locus, by Fedoroff et al. (Cell 35:235, 1983), has shown that these Ac and Ds elements differ by the deletion of about 200 bases from the central region of the Ac element. Restriction mapping of the wx-m6 and sh-m5933 Ds elements have shown them to be similar to the Ac element, but lacking about 2 kb of DNA from the central region of Ac (Fedoroff et al., 1983; Courage-Tebbe et al. Cell 34:383, 1983). These observations seem to support the proposition that these Ds elements are derivatives of Ac that arose from internal deletions that abolish the transposase function. However, examination of the data presented in the Fedoroff et al. paper on the molecular analyses of wx-m9Ac and wx-m9Ds led me to suggest an alternative scheme for the relationship between Ac and Ds. I propose that Ac is an insertional derivative of Ds, resulting from an insertion of an element equivalent to the Ac central portion into a Ds element.
The maize genome contains multiple sequences that are homologous to the wx-m6, sh-m5933 Ds elements. At least 20 discrete bands are detected that hybridize to Ds probes. Similarly, Fedoroff et al. (1983) have shown that multiple copies of the central element of Ac are also present, even in material that does not exhibit Ac function. When they digested DNA from various maize stocks with restriction enzymes that yielded internal fragments of Ac and probed with an internal fragment, up to 8-10 copies of a fragment were detected which co-migrated with the fragment of a cloned Ac element. However, when the DNA was digested with restriction enzymes that yielded fragments containing a portion of the terminal (Ds) sequence in addition to a segment of the internal sequence that overlapped the internal probes, only a single copy of a fragment that co-migrated with the fragment from a similar digest of a cloned Ac element was detected in plants which carried a single wx-m9Ac allele. The wx-m9Ds plants did not yield this fragment, but instead gave a slightly smaller fragment that co-migrated with the fragment of a cloned wx-m9Ds allele.
Thus, the two components of Ac may exist as separable elements in multiple copies, and transposase functions in effecting transposition limited to the unique composite structure where the central element exists as an insertion in a Ds element.
The alternative schemes for the relationship between Ac and Ds elements lead to strikingly different predictions. If the multiple Ds elements are defective Ac's they should have sequence homology to both the Ds probe and the central portion of Ac, except where the entire central portion had been deleted. Thus, in Southern Blot analyses, most bands that are lit up by the Ds probe should also show hybridization with the Ac internal probe. If, on the other hand, Ac is an insertional derivative of Ds, restriction fragments which contain the Ds elements should not also contain sequences homologous to the Ac central element. Fedoroff et al. digested DNA from a number of maize lines with BstE II, and probed with both an Ava I-Eco RI fragment of the central element and the Ava I fragment of the pDs6 plasmid, which contains most of the Ds element. The results presented in Figure 7 of their paper support the proposition that Ac is an insertional derivative of Ds.
Ac elements may play a role in evolution by causing gross changes in genetic make-up via chromosomal rearrangements, and it is exciting to speculate that the two "stable" basic components of this potentially highly unstable system are present in all maize strains and conjoin under conditions of environmental stress.
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