The Bg transposable element from Zea mays L. exhibits sequence homology with plant and Drosophila transposons

--H. Hartings, N. Lazzaroni, V. Rossi, M. Motto and F. Salamini

The controlling element system consisting of the two elements Bg and rbg (receptor of Bg) was originally shown to control the mutability of the o2-m(r) allele (Salamini, F, Mol. Gen. Genet. 179:497-507, 1980; Salamini, Cold Spring Harbor Symp. Quant. Biol. 45:467-476, 1981). The autonomous Bg element encodes functions required for transposition, while the non-autonomous rbg elements are deletion derivatives of the autonomous element which still bear the termini of Bg. rbg elements cannot promote their own excision, rather they transpose only in the presence of an active Bg element.

Both the autonomous Bg element and the receptor element have been isolated: the former at the waxy locus, the latter at the opaque-2 locus (Hartings, H et al., Mol. Gen. Genet. 227:91-96, 1991). The characterization of the elements at the sequence level has revealed a number of features. The elements generate an 8bp direct duplication of the target site upon transposition, and carry 5bp long terminal inverted repeats (CAGGG). The terminal repeats start with the nucleotides CA, like the repeats of other transposable elements, and in particular the Ac transposable element from maize. Similar inverted repeats, 8bp long target site duplications and the absence of a recognizable promoter sequence in the 5' part of the element are the major characteristics that Bg shares with the Ac transposable element. A direct comparison of the nucleotide sequences of the Bg and Ac elements however, did not reveal significant homologies.

Recently, Calvi et al. (Cell 66:465-471, 1991) and Feldmar and Kunze (EMBO J. 13:4003-4010, 1991) showed that the Ac ORFa protein sequence contains regions exhibiting homology with the deduced protein sequences of the Tam3 transposable element from Antirrhinum majus and the hobo element from Drosophila melanogaster. The highest homology was found near the C-terminus of the proteins.

A computer search for coding regions, based on codon frequency statistics carried out on the entire Bg sequence, revealed the presence of a number of putative coding regions in the Bg element. These putative coding regions were screened for homology with a 57 amino acid long stretch, present in the C-terminal region of the hobo element, and showing high homology with the Tam3 and Ac sequences, as indicated by Calvi and coworkers. Significant homology was found with one of the putative coding regions derived from the Bg sequence. This region encompasses nucleotides 3120 to 3959. Because no open reading frames can be found in the sequence 3' of position 3959, it is likely that the region showing homology is located in the C-terminal region of the putative Bg protein sequence. The deduced amino acid sequence of this region exhibits 31.6% of identity and 64.9% of similarity with the Ac sequence, 33.3% of identity and 66.6% of similarity with the hobo sequence, and 29.8% of identity and 66.6% of similarity with the Tam3 sequence over a stretch of 57 amino acids (Fig. 1). No other homologies were found between the Bg sequence and the protein sequences of the other transposable elements.

Figure 1. Alignment of Bg, Ac, Tam3 and hobo amino acid sequences. * indicates identities, : indicates conservative changes. For the Ac, Tam3 and hobo sequences, absolute positions are indicated.

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