Site-directed mutagenesis and phylogenetic comparisons of the Escherichia coli Tus protein: DNA-protein interactions alone can not account for Tus activity

The Tus protein of Escherichia coli is capable of arresting DNA replication in an orientation-dependent manner when bound to specific sequences in the bacterial chromosome called Ter sites. Arrest of DNA replication has been postulated to occur either by a barrier mechanism, where Tus acts as a phys ical block to replication fork progression, or through protein-protein inte ractions between Tus and some component of the replication fork. A previous mutational analysis of Tus suggested that the amino acids in the Ll loop m ight play a role in replication arrest. Site-directed mutagenesis of amino acids in the Ll loop and other amino acid residues on the "non-permissive" face of Tus was performed to identify residues that affected Tus function. One mutant, E47Q, gave results that are inconsistent with the barrier model , showing a greater affinity for the Ter site (with a t(1/2) of 348 min ver sus 150 min for wild-type Tus) but a reduced ability to arrest DNA replicat ion in vivo. In addition to the site-directed mutagenesis studies, the tus genes of Salmonella, Klebsiella, and Yersinia were sequenced and the protei ns expressed in E. coli to assess their ability to arrest DNA replication. The results presented here support a role for protein-protein interactions in Tus function, and suggest that residues E47 and E49 participate in repli cation fork arrest.