Summary Unlike nuclear multisubunit RNA polymerases I, II and III, whose subunit compositions are conserved throughout eukaryotes, plant RNA Polymerases IV and V are non-essential, Pol II-related enzymes whose subunit compositions are still evolving. Whereas Arabidopsis Pols IV and V differ from Pol II in four or five of their twelve subunits, respectively, and differ from one another in three subunits, proteomic analyses show that maize Pols IV and V differ from Pol II in six subunits, but differ from each other only in their largest subunits. Use of alternative catalytic second-subunits, which are non-redundant for development and paramutation, yields at least two subtypes of Pol IV, and three subtypes of Pol V in maize. Pol IV/V associations with MOP1, RMR1, AGO121, Zm_DRD1/CHR127, SHH2a and SHH2b extend parallels between paramutation in maize and the RNA-directed DNA methylation pathway in Arabidopsis.

The Flavr Savr tomato was introduced as the first genetically engineered whole food in 1994. The commercial event, resulting from transformation with an antisense expression cassette of the endogenous polygalacturonase gene, was sequenced and found to contain two contiguous, linked, transfer DNA insertions. We found polygalacturonase suppression correlates with accumulation of ≈21-nt small interfering RNAs, the hallmark of an RNA interference-mediated suppression mechanism.