8 Termination of DNA Replication

Sequence-specific replication termini (Ter) occur at the antipodes, i.e., 180° from the replication origins of circular bacterial chromosomes (and in some plasmid replicons, e.g., R6K), and ensure that termination occurs within a defined region. Meeting of the two forks within this region is necessary for efficient resolution of oligomers formed by an odd number of crossovers between the two daughter chromosomes, and dimer resolution is essential for proper chromosome segregation (see details in the next section). In contrast, although most eukaryotic replicons do not appear to have sequence-specific replication termini, these are present at specific regions of the chromosome that require polarized replication fork movement necessary for carrying out specific physiological functions; e.g., mating-type switching in Schizosaccharomyces pombe . This review covers the mechanism of replication fork arrest at the natural replication termini and pause sites of prokaryotes and eukaryotes and explores their possible regulatory roles in the control of replication fork movement through these regions and in other DNA transactions. Only the newer information on replication termination and fork pausing that has been uncovered since the publication of an earlier review (Bastia and Mohanty 1996) is discussed in this chapter. REPLICATION TERMINATION IN PROKARYOTES Most circular prokaryotic chromosomes terminate replication at sequence-specific termini called Ter sites. Such sites (also called Replication Fork Barriers or RFB, Replication Termination Sites or RTS, etc.) are also present at specific regions of the eukaryotic chromosomes; e.g., at the nontranscribed spacers of rDNA of most eukaryotes from yeast to man (Bastia and Mohanty 1996; Rothstein...