replication terminator tus family replication terminator tus family tus, also known as terminus utilization substance, is a protein that binds to terminator sequences and acts as a counter-helicase when it comes in contact with an advancing helicase. the bound tus protein effectively halts dna polymerase movement. tus helps end dna replication in prokaryotes. representation of the x-ray crystal structure of tus-ter protein-dna complex. (jmol rendering of coordinates from. the dna strands are shown in pink and green.) in e. coli, tus binds to 10 closely related sites encoded in the chromosome. each site is 23 base pairs. the 10 sites are called ter sites, and are designated tera, terb, ..., terj. these binding sites are asymmetric, such that when a tus-ter complex (tus protein bound to a ter site) is encountered by a replication fork from one direction, the complex is dissociated and replication continues (permissive). but when encountered from the other direction, the tus-ter complex provides a much larger kinetic barrier and halts replication (non-permissive). the multiple ter sites in the chromosome are oriented such that the two oppositely moving replication forks are both stalled in the desired termination region. bacillus subtilis utilize replication terminator protein (rtp) instead of tus. protein domains the ter protein contains two domains. the n-terminal domain is composed of an alpha helices, beta sheet, and three loops. the c-terminal domain is made of two alpha helices and one beta sheet.