Figure 6. Two General Models for RMR1 Action at the Pl1-Rhoades Allele.

RMR1 maintains nonsymmetrical methylation of the doppia element (light gray arrow) upstream of the pl1 coding region (exons in black) via an RdDM pathway. Small RNAs are produced in a RMR1-dependent fashion with homology to the doppia element, and maize orthologs of characterized RdDM proteins, as well as RMR1, then act as effectors of these siRNAs, facilitating cytosine methylation at complementary sequences of the DNA template. In the model shown in (A), the heterochromatic region of doppia is maintained and established independently of the Pl1-Rhoades chromatin state, but derepression of the upstream repetitive element in an rmr1 mutant causes changes in the nearby genic region through processivity of RNA polymerase II or other general transcription factors that bind the upstream elements. In the model shown in (B), the doppia element is repressed by the same RdDM pathway shown in (A), but the Pl′ state represents a spread of the heterochromatic domain beyond the region targeted by the siRNAs for cytosine methylation. This spread might be mediated by RMR1 activity, or by another chromatin modifier. In (B), loss of RMR1 would lead to a loss of the repressive chromatin state at doppia and the ability for it to spread.