Plasmid replication initiator RepB forms a hexamer reminiscent of ring helicases and has mobile nuclease domains

47 p.-7 fig. Boer, Roeland et alt.RepB initiates plasmid rolling-circle replication by binding to a triple 11-bp direct repeat (bind locus) and cleaving the DNA at a specific distant site located in a hairpin loop within the nic locus of the origin. The structure of native full-length RepB reveals a hexameric ring molecule, where each protomer has two domains. The origin-binding and catalytic domains show a three-layer α–β–α sandwich fold. The active site is positioned at one of the faces of the β-sheet and coordinates a Mn2+ ion at short distance from the essential nucleophilic Y99. The oligomerization domains (ODs), each consisting of four α-helices, together define a compact ring with a central channel, a feature found in ring helicases. The toroidal arrangement of RepB suggests that, similar to ring helicases, it encircles one of the DNA strands during replication to confer processivity to the replisome complex. The catalytic domains appear to be highly mobile with respect to ODs. This mobility may account for the adaptation of the protein to two distinct DNA recognition sitesThis study was supported by the Spanish Ministerio de Ciencia e Innovación (Grants BFU2005-06758/BMC and BFU2008-02372 to MC, BIO2006-02668 to FXGR, BFU2007-63575 to GdS, SAF2005-00775/SAF2008-00451 to OL, BFU2007-65977 to PC and CSD00013 to ME), the Generalitat de Catalunya (Grant 2005SGR-00280 to MC), the Fundació La Marató de TV3 (Grant 052810 to MC), the Comunidad de Madrid (Grants S-BIO-0214-2006 to OL and PC and CM-BIO0260-2006 to ME), the Red Temática de Investigación Cooperativa en Cáncer (Grant RD06/0020/1001 to OL), the Human Frontiers Science Program (Grant RGP39/2008 to OL and PC) and the EU (Spine2-Complexes LSHG-2006-031220 and 3D-Repertoire LSHG-CT-2005-512028 projects). Synchrotron data collection was supported by the ESRF and the EUPeer reviewe