“…The overall topology structure of the extended kissing complex is similar to some other RNA four-way junctions (Krol et al+, 1990;Walter et al+, 1998a;Nowakowski et al+, 1999) and DNA Holliday junctions (Duckett et al+, 1992)+ In both DNA and RNA four-way junctions, divalent ions are required for the formation and stabilization of antiparallel X-shaped structures (Duckett et al+, 1992;Walter et al+, 1998b)+ The particularity of the proposed CopA-CopT structure is the crossing over of the strands at the junction under the constraints imposed by the two loops connecting intermolecular helices B and B9+ This forces a side-by-side alignment of the two helical domains that brings the 59 tail of CopA in close proximity to the complementary region of CopT (Fig+ 7)+ The formation of intermolecular helix C, which clamps the two long helical domains, greatly enhances the stability of the complex (Persson et al+, 1990a;Malmgren et al+, 1997)+ Crystallographic analysis of a group I ribozyme domain revealed a similar organization (Cate et al+, 1996): a sharp bend induced by an internal loop allows a side-by-side alignment of two helical domains that is additionally stabilized by metal-and ribosemediated backbone contacts and two long-range tertiary interactions+ A side-by-side configuration was also proposed for the hairpin ribozyme, here stabilized by interactions between two internal loops (Earnshaw et al+, 1997)+ The formation of a stable RNA-RNA complex is not unique to CopA-CopT, and is also a key feature in the replication control of plasmids belonging to the IncB and IncIa groups (Siemering et al+, 1994; plasmids+ In these systems, the antisense RNAs inhibit the formation of a pseudoknot structure that activates rep translation (Wilson et al+, 1993;)+ All these antisense and target RNAs are characterized by stable hairpins with identical loop sequences and bulged residues in the upper stem regions+ Enzymatic probing performed on (antisense) RNAI in pMU720 plasmid bound to its target indicated that a full duplex was not rapidly formed in vitro+ Instead, binding resulted in an extended kissing complex stabilized by 59 tail interactions (Siemering et al+, 1994)+ One may therefore speculate that, in all these systems, the final product of the binding reaction in vitro is characterized by an overall topology very similar to that reported here, except that the lengths of helices B and B9, if formed in the IncB/IncIa cases, could be different+…”