Features of replication fork blockage by the Escherichia coli terminus-binding protein

Blockage of the progress of a DNA replication fork in Escherichia coli can be ascribed to an inhibition of helicase action at the orientation-specific binding of a termination sequence (ter) by the ter-binding protein (Lee, E.H., Kornberg, A., Hidaka, M., Kobayashi, T., and Horiuchi, T. (1989) Proc. Natl. Acad. Sci. U.S.A. 86, 9104-9108). These observations have been extended to include the PriA helicase, thus confirming that blockage is general for helicases. The site of arrest of synthesis by a replication fork is at the very first nucleotide of the 22-base pair E. coli-terB sequence. Strand displacement by DNA polymerases is also inhibited, but is less profound and is orientation-specific. The ter sequences of plasmids R1-terR and -terL and of plasmids R6K and R100 have been compared with those of E. coli-terA and -terB.