Southern blot analysis found one deletion in the sample, bz'-10; this derivative was cloned into the vector pUC119 after the sequence was amplified by the polymerase chain reaction. Three independent clones were sequenced using Sequenase (US Biochemicals). The bz'-10 allele is a 49 bp deletion of Bz1 sequences; the deletion begins in the 3' end of the intron, removes the dSpm and Bz1 sequences between the breakpoints, and extends 8 to 11 bp downstream of the insertion site (Figure 1). Two unusual features of this deletion were immediately apparent. First, the deletion extends to both sides of the dSpm insertion site; other transposable element-associated deletions extend to one side of the insertion site. Second, the deletion breakpoints are within a 4 bp direct repeat. Short direct repeats are often found at deletion breakpoints in spontaneous deletion alleles in bacteria and other organisms, but they are not usually associated with transposable element deletions in maize. It is possible that bz'-10 is a spontaneous deletion.
Figure 1. Sequence of bz-m13 and two stable bz1 derivatives. The top line shows the sequence from Bz1 surrounding the dSpm insertion site; the three bases duplicated by the insertion are underlined. The second line gives the sequence from bz-m13 flanking the insertion site and the sequences spanning the deletion breakpoints in bz'-10. Intron sequences are in lowercase letters, and the 4 basepair direct repeats are underlined with dotted lines. In bz'-10, the deletion begins in the 3' splice acceptor site and extends to 8-11 basepairs downstream of the dSpm insertion site. This deletion also maintains a 3' splice acceptor site, however the message is out of frame. In bz'-9, the 4 base pairs inserted are underlined.
The remaining 12 derivatives were indistinguishable
from the progenitor allele by Southern blot analysis. One of these alleles,
bz'-9, was cloned and sequenced. This derivative has a 4 bp insertion
relative to Bz1 sequences; the 3 bp host sequence duplication and
one additional base are left behind. This sequence is similar to other
stable derivative alleles examined by others.
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