Developmental regulation of excision timing ofMutator transposons of maize: Comparison of standard lines and an early excisionbzl::Mu1 line

Characterization of the germinal and somatic activity of the Arabidopsis transposable element Tag1

Tag1 is an autonomous transposon of Arabidopsis thaliana. The excision behavior of Tag1 during reproductive and vegetative development was examined using CaMV 35STag1-GUS constructs. Germinal reversion frequencies varied from 0 to 27% and correlated with Tag1 copy number. Southern blot and somatic sector analyses indicated that each revertant was derived from an independent excision event, and approximately 75% of the revertants had new Tag1 insertions. Revertants were obtained with similar frequencies from the male and female parents. In flowers, small somatic sectors were observed in siliques, carpels, petals and sepals while stemlike organs (filaments and pedicels) had larger sectors. No sectors encompassing entire flowers or inflorescences were observed, however, indicating that excision occurs late in flower development and rarely in inflorescence meristems. Late excision was also observed during vegetative development with 99.8% of leaves showing small sectors encompassing no more than 20 cells. Roots and cotyledons, however, showed larger sectors that included entire lateral roots and cotyledons. These results indicate that Tag1 can excise in the embryo and all the organs of the plant with the timing of excision being restricted to late stages of vegetative and reproductive development in the shoot.

Large tandem duplication associated with a Mu2 insertion in Zea mays B-Peru gene

The b locus of Zea mays encodes a transcriptional activator of the anthocyanin biosynthetic pathway. The B-Peru allele is expressed in the aleurone layer of the seed, which results in dark purple pigmentation of this tissue. An unstable Mutator-induced B-Peru mutant allele, b-Perum220, displays weak, variable pigment and a high germinal reversion rate not characteristic of other Mutator insertions. Characterization of relevant regions of b-Perum220 revealed a Mu2 element insertion in one copy of a 534 bp sequence. This 534 bp sequence is tandemly triplicated in the progenitor B-Peru allele, upstream of the B-Peru transcription start site. In addition to the Mu2 insertion, the b-Perum220 allele contains a newly formed large tandem duplication of 4.0 kb, which includes the promoter region and the first three exons of the B-Peru gene. The Mu2 element does not reside at any of the duplication breakpoints. The molecular study of eleven independent germinal revertants revealed five structural classes including structures in which the 4.0 kb tandem duplication is partially or completely deleted, the Mu2 element is partially or completely deleted, or a combination of these events has occurred. We hypothesize that most of the revertants arose by unequal recombination between the duplicated regions. Based on these structural analyses, models are discussed to explain the reduced b gene expression in b-Perum220.