The structure of the paramutable R locus is shown here and appears to have been derived by several steps of duplication. The functional genes are P, S1 and S2; q is a piece of a gene. The sigma component of this locus is the relic of an ancient transposon that belongs to the same family of transposons as Spm and it is now the promoter sequence for the S genes, which are divergently transcribed. The critical points are these:
1. Paramutation of this locus results in methylation of all of the promoter sequences.
2. This locus, when paramutated and methylated itself becomes paramutagenic.
3. If this central region containing the promoters is deleted, this locus loses its paramutability: it can neither be methylated nor can it become paramutagenic.