INACTIVATION OF GENE-EXPRESSION IN PLANTS AS A CONSEQUENCE OF SPECIFIC SEQUENCE DUPLICATION

Numerous examples now exist in plants where the insertion of multiple copies of a transgene leads to loss of expression of some or all copie s of the transgene. Where the transgene contains sequences homologous to an endogenous gene, expression of both transgene and endogenous gen e is sometimes found to be impaired. Several examples of these phenome na displaying different features are reviewed. Possible explanations f or the observed phenomena are outlined, drawing on known cellular proc esses in Drosophila, fungi, and mammals as well as plants. It is hypot hesized that duplicated sequences can, under certain circumstances, be come involved in cycles of hybrid chromatin formation or other process es that generate the potential for modification of inherited chromatin structure and cytosine methylation patterns. These epigenetic changes could lead to altered transcription rates or altered efficiencies of mRNA maturation and export from the nucleus. Where the loss of gene ex pression is posttranscriptional, antisense RNA could be formed on accu mulated, inefficiently processed RNAs by an RNA-dependent RNA polymera se or from a chromosomal promoter and cause the observed loss of homol ogous mRNAs and possibly the modification of homologous genes. It is s uggested that the mechanisms evolved to help silence the many copies o f transposable elements in plants. Multicopy genes that are part of th e normal gene catalog of a plant species must have evolved to avoid th ese silencing mechanisms or their consequences.