Natacha Bies‐Etheve

In order to identify new factors involved in Em (a class I Late Embryogenesis Abundant protein) gene expression, Arabidopsis mutants with an altered expression of an Em promoter GUS fusion construct and a modified accumulation of Em transcripts and proteins were isolated. Germination tests on ABA showed that the most affected mutant had a weak abi phenotype. Complementation tests further revealed this mutant to be a new abi5 allele, consequently named abi5-5. In addition to reducing the final level of Em transcripts in the dry seed, the abi5-5 mutation causes a delay in the accumulation of AtEm1 during seed development. An additional characteristic of the abi5-5 mutant, is the ability of its seeds to germinate at high concentrations of salt and mannitol. The abi5-5 mutation was characterized at the molecular level and was shown to result from a two base pair deletion in the coding sequence of the ABI 5 gene. The wild type and mutant recombinant proteins were produced in E. coli and were assayed for DNA-binding activity on their target promoters by electrophoretic mobility shift assay (EMSA). The ABI5 recombinant protein binds the ABRE sequence in the AtEm6 promoter as shown by Dnase footprinting. Among the ABRE-type sequences selected on both Em promoters, the G-box type AGACACGTGGCATGT element of the AtEm6 promoter shows the strongest binding by EMSA quantification.

RNA polymerase V (Pol V) long noncoding RNAs (lncRNAs) have been proposed to guide ARGONAUTE4 (AGO4) to chromatin in RNA-directed DNA methylation (RdDM) in plants. Here, we provide evidence, based on laser UV-assisted zero-length cross-linking, for functionally relevant AGO4-DNA interaction at RdDM targets. We further demonstrate that Pol V lncRNAs or the act of their transcription are required to lock Pol V holoenzyme into a stable DNA-bound state that allows AGO4 recruitment via redundant glycine-tryptophan/tryptophan-glycine AGO hook motifs present on both Pol V and its associated factor, SPT5L. We propose a model in which AGO4-DNA interaction could be responsible for the unique specificities of RdDM.