Tus and the terminators: The arrest of replication in prokaryotes

Bidirectional replication of prokaryotic circular chromosomes concludes when the two replication forks converge opposite the origin of replication in a region called the terminus. The terminus regions of several replicons contain sites referred to as terminators that arrest DNA replication and prevent replication forks from exiting the terminus. Function of the terminators is absolutely dependent upon a DNA binding protein that specifically binds the terminator sites and impedes replication forks. Studies on prokaryotic termination systems have now progressed to the point where termination sites have been sequenced and DNA binding proteins have been purified to near homogeneity. Very recently, several laboratories have completed the initial biochemical characterization of the mechanism of replication fork arrest, and one of these papers appears in this issue of cell. The studies that elucidated the components of the termination systems in Escherichia coli, Bacillus subtiiis, and the R6K plasmid will be reviewed here, as well as the more recent mechanistic studies. Tsrmlnator Sites The terminus of the E. coli chromosome is a large region (~360 kb) flanked on both sides by terminator sites, WA, Ter8, TerC, and TerD, that arrest replication forks (see figure; nomenclature for these sites is presented in the footnote). Before proceeding to the details of this inhibition of replication, the role of these sites in the replication cycle should be considered. inhibition is polar, and the important concept was developed that the terminus region functions as a replication-fork trap (Hill et al., 1987; deMassy et al., 1987). Specifically, replication forks from either direction are not prevented from entering the terminus region, but they are inhibited when they progress across the terminus and reach the other side. With respect to bidirectional replication of the chromosome, this means that if the two forks