The contrahelicase activities of the replication terminator proteins of Escherichia coli and Bacillus subtilis are helicase-specific and impede both helicase translocation and authentic DNA unwinding.

Replication forks are arrested at sequence-specific replication termini primarily, perhaps exclusively, by polar arrest of helicase-catalyzed DNA unwinding by the terminator protein. The mechanism of this arrest is of considerable interest. This paper presents experimental evidence in support of four major points pertaining to termination of DNA replication. First, the replication terminator proteins of both Escherichia coli and Bacillus subtilis are helicase-specific contrahelicases, i.e. the proteins specifically impede the activities of helicases that are involved in symmetric DNA replication but not of those involved in conjugative DNA transfer and rolling circle replication. Second, the terminator protein (Ter) of E. coli blocks not only helicase translocation but also authentic DNA unwinding. Third, the replication terminator protein of Gram-positive B. subtilis is a polar contrahelicase of the primosomal helicase PriA of Gram-negative E. coli. Finally, the blockage of PriA-catalyzed DNA unwinding was abrogated by the passage of an RNA transcript through the replication terminator protein-terminus complex. These results are significant because of their relevance to the mechanistic aspects of replication termination.