DNA bending of pSC101 ori induced by Rep, a replication initiator protein and IHF.

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Purified Rep (or RepA) protein, a replication initiator of plasmid pSC101, is present almost solely in the dimer form, and its binding activity for the directly repeated sequences (iterons) in the replication origin (ori) is very low. When Rep protein was treated with guanidine hydrochloride followed by renaturation, it was shown to bind to the iterons with very high efficiency. A gel shift experiment suggested that guanidine-treated Rep bound to iterons as a monomer form. The Rep monomer bound noncooperatively to the three iterons and induced bending of the DNA helix axis in the same direction (about 100°). The configuration of the IHF box that is a binding site of another DNA bending protein IHF, the three iterons and an AT-rich region between these sequences was important for efficient bending of the ori region. Furthermore, a mutant Rep protein (RepIHF) which can support the plasmid replication in IHF-deficient host cells was purified, and it was found that affinity of the RepIHF monomer for iterons was similar to that of wild-type Rep and bent DNA only 14° more strongly than did the wild-type Rep. RepIHF-dependent plasmid replication, however, required both enhancer regions, par and IR-1, in addition to "core ori" as a minimal essential ori, whereas only one of these two enhancers was necessary for wild-type Rep-dependent replication. How RepIHF can support plasmid replication in the absence of IHF is discussed.

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