This project will investigate a new hypothesis regarding how the mobile genetic element (SCCmec) that carries methicillin resistance spreads among S. aureus strains: We propose that it encodes previously unsuspected functionality for its own replication, which would enhance the efficiency of any horizontal gene transfer mechanism. MRSA (methicillin resistant S. aureus) is a serious and growing public health problem. The recombinases that integrate and excise the SCCmec element into and out of the host genome have been identified and are under study in our lab. However, nothing is known about the events between excision from one genome and appearance in another. While analyzing the conserved ORFs that flank recombinase genes, we realized that some of are likely to have functions in DNA replication. This proposal focuses on a large putative ATPase (Cch) that we discovered is related to the known replication initiation proteins encoded by S. aureus pathogenicity islands (SaPIs). Our preliminary structural and biochemical data suggest that the SCCmec protein in question (Cch) is a self-loading helicase whose closest homologs in the ATPase domain are, surprisingly, the eukaryotic and archaeal replicative MCM helicases. Thus we have determined the first structure of that type of helicase in the active ring form. This work will change how people think about SCCmec elements and how they spread methicillin resistance and will provide new model systems for understanding mechanisms of self-loading and MCM-like helicases. Aim 1: Structural understanding of Cch Aim 2: How structurally similar / different are the subtypes of SaPI / SCC replication initiator? Aim 3: What are the biochemical activities of Cch and SaPI3 rep?