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PLANT PHYSIOLOGY , Vol 115, Issue 2 361-373, Copyright © 1997 by American Society of Plant Biologists
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GENE REGULATION AND MOLECULAR GENETICS |
Epigenetic Transcriptional Silencing and 5-Azacytidine-Mediated Reactivation of a Complex Transgene in Rice
S. P. Kumpatla, W. Teng, W. G. Buchholz and T. C. Hall
Institute of Developmental and Molecular Biology, and Department of Biology, Texas A&M University, College Station, Texas 77843-3155
Despite a growing number of reports indicating non-Mendelian inheritance of
transgene expression in monocots, no detailed description of the structure
and stability of the transgene exists for transformants generated by direct
DNA-transfer techniques, making the cause for these observations difficult
to determine. In this paper we describe the complex organization of Btt
cryIIIA and bar transgenes in rice (Oryza sativa L.) that displayed
aberrant segregation in R1 progeny. Silencing rather than rearrangement of
the bar gene was implicated because the herbicide-sensitive R1 plants had a
DNA hybridization profile identical to that of the resistant R0 parent and
R1 siblings. Genomic DNA analysis revealed substantial methylation of the
Ubi1/bar sequences in silenced plants and, to a lesser degree, in
herbicide-resistant plants, suggesting that the transgene locus was
potentiated for silencing. Nuclease protection and nuclear run-on assays
confirmed that silencing was due to transcriptional inactivation. Treatment
of R2 progeny of silenced plants with 5-azacytidine resulted in
demethylation of the Ubi1 promoter and reactivation of bar gene expression,
demonstrating a functional relationship for methylation in gene silencing.
These findings indicate that methylation-based silencing may be frequent in
cereals transformed by direct DNA protocols that insert multiple, often
rearranged sequences.
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