THE REPLICATION TERMINATOR PROTEIN OF THE GRAM-POSITIVE BACTERIUM BACILLUS-SUBTILIS FUNCTIONS AS A POLAR CONTRAHELICASE IN GRAM-NEGATIVE ESCHERICHIA-COLI

The replication terminator protein (RTP) of Bacillus subtilis is a dim er with a monomeric molecular mass of 14.5 kDa. The protein terminates DNA replication at a specific binding site. Although the protein has been crystallized and its crystal structure has been solved, the lack of an in vitro replication system in B. subtilis has been a serious im pediment to the analysis of the mechanism of action of this protein. W e have discovered that the protein is functional in the Gramnegative b acterium Escherichia coil in vivo and in vitro. RTP blocked replicatio n forks initiated from a ColE1 replication origin at the cognate DNA-b inding site (BS3) in a polar mode. The protein did not block rolling c ircle replication initiated from the pT181 origin in cell extracts of Staphylococcus aureus. RTP antagonized the helicase activity of DnaB b ut not that of helicase II of E. coil. Thus, RTP functioned as a polar contrahelicase blocking a helicase that participates in symmetric DNA replication but it did not impede rolling circle replication nor the action of a helicase involved in DNA repair.