TnpA trans-activates methylated maize Suppressor-mutator transposable elements in transgenic tobacco

The maize Suppressor-mutator (Spm) transposable element is subject to epigenetic inactivation in transgenic tobacco, as it is in maize. Spm inactivation in tobacco is correlated with increased methylation of sequences near the element's transcription start site. To determine whether element-encoded gene products can promote the reactivation of an inactive element, we investigated the effects of introducing individual CaMV 35S promoter-driven cDNAs for tnpA, tnpB, tnpC and tnpD, the element's four known protein-coding sequences. Introduction of the tnpA cDNA promoted the reactivation of the inactive resident Spm element, as judged by the appearance of regenerants with very early excision events and transposed elements. By contrast, the tnpB, tnpC and tnpD cDNAs had no affect on the activity of the resident Spm element. Similar results were obtained when the element-encoded cDNAs were introduced either by Agrobacterium-mediated retransformation or by a genetic cross. Reactivation of an inactive Spm by the tnpA cDNA is accompanied by reduced methylation of several methylation-sensitive restriction sites near the element's transcription start site. Maintenance of the reactivated Spm element in an active state requires the continued presence of the tnpA cDNA. Elimination of the tnpA cDNA locus by genetic segregation generally results in decreased element activity, as judged by a low frequency of excision events, and is accompanied by increased methylation of the element's 5'-end. Exceptions resembling the phenomenon of "presetting" are also observed in which progeny plants that did not receive the tnpA cDNA locus after meiotic segregation maintain high excision activity and exhibit low methylation levels.