Diversity of Pol IV Function Is Defined by Mutations at the Maize rmr7 Locus

Mutations affecting the heritable maintenance of epigenetic states in maize identify multiple small RNA biogenesis factors including NRPD1, the largest subunit of the presumed maize Pol IV holoenzyme. Here we show that mutations defining the required to maintain repression7 locus identify a second RNA polymerase subunit related to Arabidopsis NRPD2a, the sole second largest subunit shared between Arabidopsis Pol IV and Pol V. A phylogenetic analysis shows that, in contrast to representative eudicots, grasses have retained duplicate loci capable of producing functional NRPD2-like proteins, which is indicative of increased RNA polymerase diversity in grasses relative to eudicots. Together with comparisons of rmr7 mutant plant phenotypes and their effects on the maintenance of epigenetic states with parallel analyses of NRPD1 defects, our results imply that maize utilizes multiple functional NRPD2-like proteins. Despite the observation that RMR7/NRPD2, like NRPD1, is required for the accumulation of most siRNAs, our data indicate that different Pol IV isoforms play distinct roles in the maintenance of meiotically-heritable epigenetic information in the grasses.     

Identification and characterisation of an RPD3 homologue from maize (Zea mays L.) that is able to complement an rpd3 null mutant of Saccharomycescerevisiae

In mammals, yeast and Drosophila, the histone deacetylase RPD3 proteins can alter the expression of genes involved in fundamental biological processes by affecting the degree of acetylation of histones and changing chromatin structure. Here we report the isolation of a cDNA sequence encoding an RPD3 homologue from maize, which is able to complement the phenotype of an rpd3 null mutant of the yeast Saccharomyces cerevisiae. The expression of the corresponding gene(s) was assessed in different maize tissues. The number of homologous loci was estimated by Southern hybridisation to be in the range of two to three, and the chromosomal location of one of these loci was determined. Phylogenetic analysis and tests for relative divergence rates, using related RPD3 sequences from different species, were performed, and suggest that different polymorphic forms of RPD3-like proteins that evolve at distinct rates are present in the species considered.