The exosome is the major 3'-5' RNA-degradation complex in eukaryotes. The ubiquitous core of the yeast exosome (Exo-10) is formed by nine catalytically inert subunits (Exo-9) and a single active RNase, Rrp44. In the nucleus, the Exo-10 core recruits another nuclease, Rrp6. Here we crystallized an approximately 440-kilodalton complex of Saccharomyces cerevisiae Exo-10 bound to a carboxy-terminal region of Rrp6 and to an RNA duplex with a 3'-overhang of 31 ribonucleotides. The 2.8 Å resolution structure shows how RNA is funnelled into the Exo-9 channel in a single-stranded conformation by an unwinding pore. Rrp44 adopts a closed conformation and captures the RNA 3'-end that exits from the side of Exo-9. Exo-9 subunits bind RNA with sequence-unspecific interactions reminiscent of archaeal exosomes. The substrate binding and channelling mechanisms of 3'-5' RNA degradation complexes are conserved in all kingdoms of life.

Processive chromosomal replication relies on sliding DNA clamps, which are loaded onto DNA by pentameric clamp loader complexes belonging to the AAA+ family of ATPases. We present structures for the ATP-bound state of the clamp loader complex from bacteriophage T4, bound to an open clamp and primer-template DNA. The clamp loader traps a spiral conformation of the open clamp so that both the loader and the clamp match the helical symmetry of DNA. One structure reveals that ATP has been hydrolyzed in one subunit, and suggests that clamp closure and ejection of the loader involves disruption of the ATP-dependent match in symmetry. The structures explain how synergy between the loader, the clamp and DNA can trigger ATP hydrolysis and release of the closed clamp on DNA.

Summary Clamp loaders load sliding clamps onto primer-template DNA. The structure of the E. coli clamp loader bound to DNA reveals the formation of an ATP-dependent spiral of ATPase domains that tracks only the template strand, allowing recognition of both RNA and DNA primers. Unlike hexameric helicases, in which DNA translocation requires distinct conformations of the ATPase domains, the clamp loader spiral is symmetric and is set up to trigger release upon DNA recognition. Specificity for primed DNA arises from blockage of the end of the primer and accommodation of the emerging template along a surface groove. A related structure reveals how the ψ protein, essential for coupling the clamp loader to single-stranded DNA binding protein (SSB), binds to the clamp loader. By stabilizing a conformation of the clamp loader that is consistent with the ATPase spiral observed upon DNA binding, ψ binding promotes the clamp loading activity of the complex.