Control of ColE1 Plasmid replication: Interaction of rom protein with an unstable complex formed by RNA I and RNA II

Abstract

A transcript (RNA I) from ColE1 inhibits initiation of replication of the plasmid DNA by binding to the precursor of the primer RNA (RNA II). The ability of RNA I to inhibit replication is altered by the presence of a plasmid-specified small protein, Rom. In vitro, RNA I binds to RNA II to form a very unstable complex, C ∗. Binding of a single molecule of Rom converts C ∗ to a more stable complex, C m∗. Each of these complexes, C ∗ or C m∗, transforms to a more stable complex, C ∗∗ or C m∗∗, respectively. While formation of complex C ∗ or C m∗ is inferred from the inhibition of binding caused by a second RNA I species, that of complex C ∗∗ or C m∗∗ is detected by alteration of RNase sensitivity. Complex C ∗ converts to complex C m∗ very rapidly upon addition of Rom to the medium and complex C m∗ converts to complex C ∗ very rapidly by removal of Rom from the medium. On the other hand, complexes C ∗∗ and C m∗∗ do not rapidly interconvert, but can eventually transform to the same stable final product. Thus, Rom affects binding of RNA I to RNA II through conversion of a very unstable early intermediate to a more stable complex, creating a second pathway for their stable binding.