DNA-REPLICATION FORK BLOCKED AT THE TER SITE MAY BE AN ENTRANCE FOR THE RECBCD ENZYME INTO DUPLEX DNA

In Escherichia coil, eight kinds of chromosome-derived DNA fragments ( named Hot DNA) were found to exhibit homologous recombinational hotspo t activity, with the following properties. (i) The Hot activities of a ll Hot DNAs were enhanced extensively under RNase H-defective (mh) con ditions. (ii) Seven Hot DNAs were clustered at the DNA replication ter minus region on the E. coli chromosome and had Chi activities (H. Nish itani, M. Hidaka, and T. Horiuchi, Mel. Gen. Genet. 240:307-314, 1993) . Hot activities of HotA, -B, and -C, the locations of which were clos e to three DNA replication terminus sites, the TerB, -A, and -C sites, respectively, disappeared when terminus-binding (Tau or Tus) protein was defective, thereby suggesting that their Hot activities are termin ation event dependent. Other Hot groups showed termination-independent Hot activities. In addition, at least HotA activity proved to be depe ndent on a Chi sequence, because mutational destruction of the Chi seq uence on the HotA DNA fragment resulted in disappearance of the HotA a ctivity. The HotA activity,which had disappeared was reactivated by in sertion of a new, property oriented Chi sequence at the position betwe en the HotA DNA and the TerB site. On the basis of these observations and positional and orientational relationships between the Chi and the Ter sequences, we propose a model in which the DNA replication fork b locked at the Ter site provides an entrance for the RecBCD enzyme into duplex DNA.