A structural-maintenance-of-chromosomes hinge domain-containing protein is required for RNA-directed DNA methylation

RNA-directed DNA methylation (RdDM) is a process in which dicer-generated small RNAs guide de novo cytosine methylation at the homologous DNA region. To identify components of the RdDM machinery important for Arabidopsis thaliana development, we targeted an enhancer active in meristems for methylation, which resulted in silencing of a downstream GFP reporter gene. This silencing system also features secondary siRNAs, which trigger methylation that spreads beyond the targeted enhancer region. A screen for mutants defective in meristem silencing and enhancer methylation retrieved six dms complementation groups, which included the known factors DRD1 (ref. 3; a SNF2-like chromatin-remodeling protein) and Pol IVb subunits. Additionally, we identified a previously unknown gene DMS3 (At3g49250), encoding a protein similar to the hinge-domain region of structural maintenance of chromosomes (SMC) proteins. This finding implicates a putative chromosome architectural protein that can potentially link nucleic acids in facilitating an RNAi-mediated epigenetic modification involving secondary siRNAs and spreading of DNA methylation.     

The cytoplasmic carboxy terminus of M13 procoat is required for the membrane insertion of its central domain

The M13 coat protein spans the Escherichia coli plasma membrane with its amino-terminus facing the periplasm. It is made as a precursor--the procoat--with a typical leader peptide. Mutations which destroy the basic character of the carboxy-terminal domain of procoat, a domain which is oriented towards the cytoplasm, block membrane assembly, while insertion of three lysyl residues near the carboxy terminus partially restores assembly. Thus the information specifying membrane insertion of M13 procoat protein is found in its mature region as well as the leader and is not simply decoded in an amino to carboxy direction.