Resumen de Functional analysis of two Arabidopsis SET domain-containing proteins (SDG7 and SDG24) during development
The Arabidopsis thaliana genome contains more than 30 genes encoding SET-domain proteins that are involved in methylating various residues of histones. The SET Domain Group (SDG) gene family can be divided into 5 major subgroups based on the homology with Drosophila SET proteins. Here, we analyzed two A. thaliana ASH1 homologs, SDG7 and SDG24, which contain a SET domain accompanied by AWS and post-SET domains. Expression analyses demonstrated that SDG7 is expressed in proliferating cells of above and underground tissues, and localized within the nucleus and cytoplasm of root apical meristem (RAM) cells. The SDG24 gene revealed the presence of several splicing variants. Protein characterization of two SDG24 isoforms, SDG24.1 and SDG24.2, showed differential expression patterns in the root and gametophyte organs. In vitro assays showed that recombinant SDG7 and SDG24.1 bind to modified histone residues, but histone methyltransferase (HMT) activity was not detected. Specific nucleolar expression of SDG24.1 was identified in RAM cells. Analysis of SDG7 and SDG24 null mutants revealed alterations in the cell division and the DNA replication pattern in the RAM, as well as major alterations of ribosomal DNA (rDNA) repeats contained in young and differentiated tissues of the sporophyte. The sdg7-5 mutant presented an increase of active rDNA copies associated with the nucleolar-organizing region of chromosome 4 (NOR4), whereas the sdg24-2 mutant displayed a loss of inactive rDNA copies associated with the NOR2 (chromosome 2). The loss of NOR2-inactive variants of sdg24-2 is epistatic to sdg7-5 and to atxr5/6 mutants (H3K27me1 HMTases) regarding the rDNA phenotype. We therefore propose that SDG7 and SDG24 have a major function in the control of NORs stability by regulating rDNA copy number.
