swi1 and swi3 Perform Imprinting, Pausing, and Termination of DNA Replication in S. pombe

The developmental program of cell-type switching of S. pombe requires a strand-specific imprinting event at the mating-type locus (mat1). Imprinting occurs only when mat1 is replicated in a specific direction and requires several trans- acting factors. This work shows (1) that the factors swi1p and swi3p act by pausing the replication fork at the imprinting site; and (2) that swi1p and swi3p are involved in termination at the mat1-proximal polar-terminator of replication (RTS1). A genetic screen to identify termination factors identified an allele that separated pausing/imprinting and termination functions of swi1p. These results suggest that swi1p and swi3p promote imprinting in novel ways both by pausing replication at mat1 and by terminating replication at RTS1.