Molecular analysis of the Bg-rbg transposable element system

--C. Spilmont, H. Hartings, N. Lazzaroni, V. Rossi, M. Motto, R. D. Thompson and F. Salamini

We have continued our work on the molecular analysis of the Bg-rbg transposable element system. An au-tonomous Bg element and a non-autonomous rbg element which inactivate, respectively, the Wx gene and the O2 gene of maize have been cloned.

The nucleotide sequence of the autonomous Bg element has been determined. This element is 4,869bp long. Moreover, sequence data obtained from the autonomous Bg and receptor element (rbg) indicate that both generate an 8-bp duplication at the insertion site upon transposition, and carry 5-bp terminal inverted repeats. The inverted repeat starts with the bases CA, like a number of other elements, and exhibits further homologies to them (Table 1).

It was also interesting to note that the 5 nucleotides forming the inverted repeats of both the Bg and rbg elements are the initial part of the 11-bp and 10-bp inverted repeats found, respectively, at the end of the Ac and dTph1 elements. Because of the preserved terminal five bases in all these elements they may be grouped into the CAGGG family; the 5-bp terminal repeats of members of this family display perfect sequence homology, and each elementgenerates an 8-bp duplication of the target site upon integration. Homology of 5bp of the terminal inverted repeats has also been used to group Tam1, Tgm1 and En/Spm into the CACTA family (Gierl and Saedler, Plant Mol. Biol. 13;261, 1989).

At 813bp from the 5' end of the Bg element an ATG codon is present. This codon is followed by a 735bp long open reading frame and ends with a TAA stop codon. The remainder of the Bg element sequence following this site contains three other potential coding regions. A 76bp long direct repeated sequence is present in the subterminal regions of the Bg element. This sequence is characterized by the presence, on both strands, of a number of perfect and imperfect copies of the hexanucleotide sequence TATCGGC. Finally, restriction enzyme analysis reveals that, compared to Bg, the receptor element is distinguishable by small deletion and insertion events. Sequence data indicate that not more than 75% of homology exists between the rbg element and the autonomous element.

Table 1.  5'---->3' termini (inverted repeats) of some transposons of a number of species

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