Rop is a small dimeric protein responsible for keeping the copy number of ColE1 family and related bacterial plasmids low in E. coli by increasing the speed of pairing between the preprimer RNA, RNA II, and its antisense RNA, RNA I. Structurally, Rop is a homodimeric four-helix bundle protein formed by the antiparallel interaction of two helix-turn-helix monomers. Wikipedia