The invention relates to bacterial genes and proteins that are implicated in the process of DNA replication and also to bacteriophage genes and their protein products that interact with bacterial proteins involved in DNA replication. More particularly, the invention relates to compositions and methods involving an essential Staphylococcus aureus gene and its encoded protein STAAU_R9. In addition, the invention relates to screening assays to identify compounds which modulate the level and/or activity of STAAU_R9 and to such compounds.