Chromosome-breaking structure in maize involving a fractured Ac element.

Active endogenous transposable elements, useful tools for gene isolation, have not been reported from any legume species. An active transposable element was suggested to reside in the W4 locus that governs flower color in soybean. Through biochemical and molecular analyses of several revertants of the w4-m allele, we have shown that the W4 locus encodes dihydroflavonol-4-reductase 2 (DFR2). w4-m has arisen through insertion of Tgm9, a 20,548-bp CACTA-like transposable element, into the second intron of DFR2. Tgm9 showed high nucleic acid sequence identity to Tgmt *. Its 59 and 39 terminal inverted repeats start with conserved CACTA sequence. The 39 subterminal region is highly repetitive. Tgm9 carries TNP1-and TNP2-like transposase genes that are expressed in the mutable line, T322 (w4-m). The element excises at a high frequency from both somatic and germinal tissues. Following excision, reinsertions of Tgm9 into the DFR2 promoter generated novel stable alleles, w4-dp (dilute purple flowers) and w4-p (pale flowers). We hypothesize that the element is fractured during transposition, and truncated versions of the element in new insertion sites cause stable mutations. The highly active endogenous transposon, Tgm9, should facilitate genomics studies specifically that relate to legume biology. Figure 1) and purple hypocotyls. Mutations at the W4 locus in the W1_ background result in altered pigment accumulation patterns in petals and reduced levels of purple pigments in flowers and hypocotyls. Four mutant alleles, w4, w4-m, w4-dp, and w4-p have been mapped to this locus. The w4 allele represents a spontaneous mutation, which produces near-white flowers (Figure 1) and green hypocotyls (Hartwig and Hinson 1962;Groose and Palmer 1991). The w4-m allele was identified from a cross between two experimental breeding lines with white and purple flowers, respectively (Palmer et al. 1989;Weigelt et al. 1990). w4-m is characterized by variegated flowers (Figure 1) and green hypocotyls with purple sectors (Groose et al. 1988).w4-m has been proposed to harbor a class II transposable element (Palmer et al. 1989). Presumably, somatic excision of the putative transposable element results in the variegated (Groose et al. 1988) and germinal excision wild-type phenotypes, purple flowers and purple pigments on hypocotyls (Palmer et al. 1989;Groose et al. 1990). The mutable line carrying w4-m undergoes germinal reversion at a very high frequency, about 6% per generation ). Approximately 1% of the progeny derived from germinal revertants contain new mutations in unlinked loci, presumably resulting from reinsertion of the element (Palmer et al. 1989). For example, female partial-sterile 1 (Fsp1), female partial-sterile 2 (Fsp2), female partialsterile 3 (Fsp3), and female partial-sterile 4 (Fsp4) were isolated from progenies of germinal revertants with purple flowers and were mapped to molecular linkage groups (MLG) C2, A2, F, and G, respectively (Kato and Palmer 2004). Similarly, 36 male-sterile, female-sterile mutants mapped to t...

Section: Discussionmentioning

confidence: 99%

Excision of an Active CACTA-Like Transposable Element From DFR2 Causes Variegated Flowers in Soybean [Glycine max (L.) Merr.]

Brar

Grosic

et al. 2010

“…State I or ''double Ds'' elements consist of nested Ds insertions or contain multiple Ds end sequences in close proximity (Doring et al , 1989(Doring et al , 1990Weck et al 1984;Ralston et al 1989;Weil and Wessler 1993). State II elements are likely deletion derivatives of Ac that differ in size and internal structure (reviewed in Kunze and Weil 2002).…”

mentioning

confidence: 99%

State IIDissociationElement Formation FollowingActivatorExcision in Maize

Conrad¹,

Bai

Ahern

et al. 2007

Active Activator (Ac) elements undergo mutations to become nonautonomous Dissociation (Ds) elements at a low frequency. To understand the mechanism of Ds formation, we have developed high-throughput genetic and molecular screens to identify these rare Ds derivatives generated from any Ac insertion in the maize genome. Using these methods we have identified 15 new Ds elements derived from Ac insertions at eight different loci. Approximately half of the Ds elements contain filler DNA inserted at the deletion junction that is derived from sequences within or adjacent to Ac. In contrast to previous reports, several of these Ds elements lack direct repeats flanking the deletion junctions and filler DNA in the donor Ac. To accommodate our findings and those of others, we propose a model of slip mispairing during error-prone repair synthesis to explain the formation of state II Ds elements in maize. We discuss the use of these lines and molecular techniques developed here to capture somatic Ds transposition events in two-component Ac/Ds tagging programs in maize.

“…As the derivative is missing the end of the element it is most similar to fAc elements described previously by Dooner (Ralston et al 1989). However, we refer to it here as a Ds element as it was derived from Ac and is no longer capable of autonomous transposition.…”

Section: Resultsmentioning

confidence: 78%

“…Some Ds elements, such as Ds9, are derived from simple internal deletions of Ac sequence (Pohlman et al 1984b). More complex structures can also be generated when one Ds element inserts into or near another Ds element (CourageTebbe et al 1983;Klein et al 1988;Doring et al 1989;Ralston et al 1989;Dooner and Belachew 1991). These complex elements are capable of inducing chromosome breakage (English et al 1995), a phenomenon first discovered by McClintock in 1946(McClintock 1946Doring et al 1989).…”

Section: T He Maize Hat Family Members Activator/dissociationmentioning

confidence: 99%

The Activator/Dissociation Transposable Elements Comprise a Two-Component Gene Regulatory Switch That Controls Endogenous Gene Expression in Maize

Bai

Brutnell

2011

The maize Activator/Dissociation (Ac/Ds) elements are able to replicate and transpose throughout the maize genome. Both elements preferentially insert into gene-rich regions altering the maize genome by creating unstable insertion alleles, stable derivative or excision alleles, or by altering the spatial or temporal regulation of gene expression. Here, we characterize an Ac insertion in the 59-UTR of the Pink Scutellum1 (Ps1) gene and five Ds derivatives generated through abortive transposition events. Characterization of Ps1 transcription initiation sites in this allelic series revealed several that began within the terminus of the Ac and Ds elements. Transcripts originating within Ds or Ac accumulated to lower levels than the wild-type Ps1 allele, but were often sufficient to rescue the seedling lethal phenotype associated with severe loss-of-function alleles. Transcription initiation sites were similar in Ac and Ds derivatives, suggesting that Ac transposase does not influence transcript initiation site selection. However, we show that Ac transposase can negatively regulate Ps1 transcript accumulation in a subset of Ds-insertion alleles resulting in a severe mutant phenotype. The role of maize transposons in gene evolution is discussed.