Cytosine methylation at CG and CNG sites is not a prerequisite for the initiation of transcriptional gene silencing in plants, but it is required for its maintenance

Transgenes integrated into plant chromosomes, and/or endogenous plant genes, may be subjected to epigenetic silencing at the transcriptional or posttranscriptional level. Transcriptional inactivation is correlated with hypermethylation of CG/CNG sites at the silent loci. It is not known whether local hypermethylation is part of the inactivation process, or just an outcome of the silent state. To address this issue, we generated transgenic tobacco lines containing a selectable marker gene controlled by a derivative of the 35S promoter of the cauliflower mosaic virus (CaMV) devoid of CG and CNG methylation acceptor sites. Silencing was triggered by crossing to the silencer locus of tobacco line 271. This line contains inactive and methylated copies of the 35S promoter and is able to silence homologous promoter copies at ectopic chromosomal positions. The mutated promoter lacking CG/CNG methylation acceptor sites was as susceptible to Trans-silencing as the unmodified 35S promoter control. Thus, methylation at CG and CNG sites is not a prerequisite for the initiation of epigenetic gene inactivation. Interestingly, while methylation of the remaining cytosines is usually only slightly affected by silencing, it was significantly increased in the absence of CG/CNG sequences. Since this sequence preference is the same as that of known methyltransferases, this may imply that silencing is accompanied or directly followed by recruitment of methyltransferase, which, in the absence of cytosines in the optimal sequence context, modifies other C residues in the affected area. However, silencing without CG/CNG methylation was immediately relieved in the absence of the silencer. Thus, CG/CNG methylation is probably essential for the maintenance of previously established epigenetic states.