Novel crystal form of the ColE1 Rom protein

The RNA I modulator protein (Rom) acts as a co-regulator of ColE1 plasmid copy number by binding to RNA kissing hairpins and stabilizing their interaction. The structure of Rom has been determined in a new crystal form from X-ray diffraction data to 2.5 Å resolution. In this structure, a dimer of the 57-amino-acid protein is found in the asymmetric unit. Each subunit consists almost entirely of two antiparallel α-­helices joined by a short hairpin bend. The dimer contains a non-crystallographic twofold axis and forms a highly regular four-α-helical bundle. The structural packing in this novel crystal form is different from previously known Rom structures. The asymmetric unit contains one dimer, giving a crystal volume per protein weight (V_M) of 1.83 ų Da⁻¹ and a low solvent content of 30%. Strong packing interactions and low solvation are characteristic of the structure. The Rom protein was cocrystallized with the Tar–Tar* kissing hairpin RNA. Although the electron-density maps do not show bound RNA, altered conformations in the side chains of Rom that are known to be involved in RNA binding have been identified. These results provide additional information about Rom protein conformational flexibility and suggest that the presence of a highly charged polymer such as RNA can promote tight packing of an RNA-binding protein, even when the RNA itself is not observed in the crystal.