Specific binding of IciA protein to the 13-mers in the origin of a minichromosome (oriC) inhibits initiation of replication in vitro by blocking the opening of this region effected by the initiator DnaA protein (Hwang, D.S., and Kornberg, A. (1990) Cell 63, 325-331). Isolation of the iciA gene (Thöny, B., Hwang, D.S., Fradkin, L., and Kornberg, A. (1991) Proc. Natl. Acad. Sci. U.S.A. 88, 4066-4070) has made possible the construction of an IciA-overproducing strain, which in turn has simplified the isolation of a large quantity of the protein, indistinguishable from that of the wild-type strain. Based on sedimentation and gel filtration, the IciA protein is an elongated dimer of a 33.4-kDa subunit. The specific binding of IciA protein to the 13-mers was stable for 2 h at 30 degrees C. The amounts of IciA protein, detected by immunoassays, increased 4-fold compared with levels (about 100 dimers) in log-phase cells whereas levels of DnaA protein decreased upon entry of cells into the stationary phase.
