Is our 'heartbreaker' a 'tourist'?
--Guri Johal, Pam Close and Steve Briggs

'Heartbreaker' is the designation given to an insertion found in one of the tagged Hm1 mutants. This mutant, hm1-1062::dHbr, was isolated during directed-mutagenesis of Hm1 by Mutator and was the first allele to be characterized in detail. A Mu1-hybridizing band was found to co-segregate with the mutant allele and was cloned. However, when mapped at higher resolution, it was found to be present ~5cM away, on the proximal side, from the Hm1 gene. This disheartening result led us to designate the hm1-1062::dHbr mutant as the heartbreaker mutant and the agent (an insertion) that caused this mutation, a 'heartbreaker'.

The Hm1 gene was eventually cloned with the help of other mutations. Southern blot hybridizations of the hm1-1062::dHbr DNA with a gene-specific probe identified a small insertion in the 3' end of the gene. A 3.1kb XhoI fragment containing the insertion was cloned and sequenced. A 314bp insertion, absent in the progenitor allele, was recognized in the mutant allele. The overall structure of this insertion was reminiscent of a transposable element but lacked homology with any known transposable element systems. This insertion has a 14-17bp imperfect terminal inverted repeat and a flanking three base pair duplication. The database search located a highly homologous (>90% identity), 314bp sequence present 3' of the wildtype waxy gene (thanks to Alfons Gierl, A for providing the complete sequence). A similar but diverged sequence was also found in the first intron of the P gene.

A highly repetitive pattern was observed on Southern blots of maize DNA but no cross hybridization was detected with sorghum and Arabidopsis DNA. A cDNA library from B73 was probed with 'heartbreaker' and 30 different cDNA clones were isolated. DNA restriction and hybridization analyses have suggested these clones to be unique and probably represent 'heartbreaker' co-transcripts since all the clones contain at least one of the TIRs.

Recently, a new transposable element has been isolated in Sue Wessler, S's laboratory which identified a new transposable element system, named 'tourist' (Bureau and Wessler, Plant Cell 4:1283, 1992). Our 'heartbreaker' seems to belong to this system. Out of the 14 bases that constitute the 'tourist' TIR, 11 are identical in 'heartbreaker'. In addition, like 'tourist', 'heartbreaker' causes a three base pair duplication of the target site and has the same target site specificity, i.e., TAA. However, the size and internal sequences are completely different in these two element families. 

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