Molecular analysis of paramutant plants of Antirrhinum majus and the involvement of transposable elements

Abstract

Paramutation is observed when the Antirrhinum majus lines 44 and 53 are crossed. These two lines both have insertions at the nivea locus, which encodes chalcone synthase (chs). The allele niv-53 carries the transposable element Tam1 in the promoter region of the chs gene; niv-44 carries the element Tam2 within the gene. The Tam1 element has previously been extensively characterised. Here the Tam2 element is further characterised, and the arrangement of the nivea locus in paramutant plants is analysed. The complete sequence of Tam2, and that of a partial cDNA complementary to it, have been determined. The cDNA is probably transcribed from a different copy of Tam2 from that present at the nivea locus, and does not encode a functional protein. Genomic Southerns of F1 plants from the 53/44 cross show that no major rearrangements are consistently associated with paramutation at the nivea locus of A. majus. The isolation from a paramutant plant arising from a 53/44 cross of an allele (niv-4432) resulting from the excision of Tam2 is reported. The excision of Tam2 resulted in a 32 bp deletion of chs gene sequences. Plants homozygous for the new niv-4432 allele have white flowers and are still paramutagenic, demonstrating that Tam2 need not be present at the nivea locus for paramutation to occur. Different interactions between Tam1 and Tam2 are discussed, and a possible model for paramutation is presented.

Pigmentation mutants produced by transposon mutagenesis in Antirrhinum majus

“…No other homologous sequences were detected, except for a 100 bp region showing about 70% homology between Taml and Tam2 at their left ends. At both ends of Taml and Tam2, a motif similar to the one described for Tam4 has been reported (Krebbers et al, 1987). As in Tam4, the motif was more abundant at the right end, with just a few copies at the other end.…”

“…This illustrates the potential of transposon mutagenesis for trapping new transposons in previously defined genes, as well as for tagging new genes. Tam4 is present in about 10 copies in the parental line JI:75 and is related to Taml and Tam2: all three transposons have identical terminal inverted repeats, produce the same length of target duplication (3 bp) and share at one end a 600-700 bp region of homology (Bonas et a/., 1984;Krebbers et a/., 1987;Sommer et a/., 1988b). The homologous region contains many palindromic pairs of a motif, and Tam4 and Tam2 have deletions of an integral number of palindromic pairs in this region relative to Taml .…”

“…The crosses may therefore affect the relative rates of somatic and germinal excision rather than the overall rates of transposition. This might also apply to the activation of Tam2 excision from niv-44, because 1 out of 37 plants from a cross involving niv-44 had lost Tam2, possibly by imprecise germinal excision in the niv-44 parent (Krebbers et al, 1987). This could be explained if Tam2 can excise in germinal cells of the niv-44 line even though somatic excision does not occur.…”

Floral homeotic mutations produced by transposon-mutagenesis in Antirrhinum majus.

“…Three different transposons (Tam/, Tam2, and Tam3), representing two distinct transposon families, have been isolated and characterized from this species (Bonas et al 1984;Sommer et al 1985;Upadhaya et al 1985;Krebbers et al 1987), and transposon-tagging has been used successfully for gene isolation (Martin et al 1985). Several genes encoding flower pigment biosynthetic enzymes have been isolated and provide good markers for monitoring transposition and for trapping new transposons (Coen et al 1986;Sommer and Saedler 1986).…”

Epigenetic Interactions among Three dTph1 Transposons in Two Homologous Chromosomes Activate a New Excision-Repair Mechanism in Petunia

“…Also, epigenetic mechanisms contribute to the regulation of transposition frequency. The variegated flower color specified by the Tam2 insertion allele nivea-44 of snapdragon is inherited in a non-Mendelian manner, suggesting that one or more epigenetic factors regulate Tam2 activity (Hudson et al, 1987;Krebbers et al, 1987). In maize, the heritable inactivation of Ac and Spm elements is associated with increased methylation and reduced transposase expression (Banks et al, 1988;Brutnell and Dellaporta, 1994).…”