The maize elements Ac/Ds are members of a widely distributed class of eukaryotic transposable elements which are characterized by a non-transcriptional transposition mechanism, terminal inverted repeats and the creation of target site duplications at their insertion sites. To date, a group of 15 transposable elements or transposable element-like sequences have been discovered whose terminal inverted repeats have sequences similar to Ac/Ds.
In addition, five of these elements contain internal sequences (putatively) encoding similar transposases. This so-called hAT family includes the plant elements Ac from Zea mays (Kunze et al., EMBO J. 6:1555-1563, 1987), Tam3 from Antirrhinum majus (Hehl et al., Plant Mol. Biol. 16:369-371, 1991) and Pac1 from Pennisetum glaucum (MacRae et al., Genetica 92:77-89, 1994), and the insect elements hobo from Drosophila melanogaster (Calvi et al., Cell 66:465-471, 1991) and Hermes from Musca domestica (Warren et al., Genet. Res., in press, 1995).
By homology searches we have determined that the autonomous element Bg from Zea mays (Hartings et al., Maydica 36:355-359, 1991) and the element Tag1 from Arabidopsis thaliana (Tsay et al., Science 260:342-344, 1993) encode amino acid sequences with strong homology to the hAT family (Figure 1). As was described for the other members, the highest degree of conservation is observed within three separate segments, termed conserved regions I, II and III. This suggests that the Bg and Tag1 transposable elements are also members of the hAT family. Two other DNAs potentially encoding proteins with a weak homology to the hAT transposases were isolated from barley genomic DNA (Chernyshev et al., Genetika 24:1338-1344, 1988) and rice cDNA (dbj RICC100171, frame +2). Moreover, by the polymerase chain reaction two sequences with homologies to conserved region I were detected in the tobacco budworm (Heliothis virescens) and the corn earworm (Helicoverpa zea) (DeVault et al., Biochem. Biophys. Res. Comm. 203:169-175, 1994). Accordingly, Ac transposase-related proteins appear to be widely distributed in many different species.
Finally we want to point out one interesting fact: The deduced amino acid sequence from the maize Bg element is more similiar to that from the Arabidopsis thaliana element Tag1 than to the maize Ac transposase (Figure 2). Furthermore, the putative coding regions of plant transposons Bg and Tag1 are more closely related to those of the insect elements hobo and Hermes than to the other plant transposons Ac, Pac1 and Tam3. The existence of a family of related (hypothetical) transposase proteins, which are required for the mobilization of transposable elements, in very distantly related species could indicate the possibility of a spreading of these elements by horizontal gene transfer. This mechanism of distribution has been suggested for the P element (Houck et al., Science 253:1125-1129, 1991) and mariner (Robertson, Nature 362:241-245, 1993).
Figure 1. Multiple alignment of conserved regions I, II and III from members of the hAT family of Ac-like transposases. For Bg and Tag1 the position numbers correspond to the codon number of the full GenBank sequence entry. The expressed frames are for Bg in region I frame +2 and in region II and III frame +3. All amino acids of Tag1 are coded in frame +3. Identical amino acids are shown inverted, amino acids similar to them are shaded grey.
2. Calculated evolutionary relationship between different Ac-like
transposases. The calculation was done with the GCG program "pileup", using
all three conserved regions shown in Figure 1.
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