A phenomenon that resembles paramutation was discovered when people began to make transgenic plants. One of the earliest reports was from Rich Jorgensen, who introduced a chalcone synthase transgene into Petunia plants and found that both the transgene and the endogenous gene could be inactivated simultaneously. The gene was commonly inactivated in part of the floral tissue, as shown in this illustration, or it could be silenced throughout the plant.
Similar observations were subsequently made by many investigators for many different transgenes. It appears that a certain fraction of transformants derived by the commonly used Agrobacterium transformation technology exhibit what has come to be called gene silencing or co-suppression. What this means is that if an extra copy of a gene already present in the genome is introduced, both the endogenous and the introduced copies are silenced together, hence the name “co-suppression.” But the gene silencing is not confined to endogenous genes. A transgene can be silenced, as well, and the probability that it is silenced increases with the number of copies. The salient features of co-suppression are these:
1. The probability of inactivation increases with copy number
2. The copies can be tandem direct repeats or inverted repeats and inverted repeats are more effective than direct repeats.
3. Silencing can be heritable, but isn’t always.
4. A silenced multicopy locus can be “paramutagenic,” meaning that it can transfer its “silenced” state to another allele of the gene located elsewhere in the genome.