Biochemical interactions between proteins and mat1 cis-acting sequences required for imprinting in fission yeast.

DNA recombination required for mating type (mat1) switching in Schizosaccharomyces pombe is initiated by mat1 imprinting. The imprinting event is regulated by mat1 cis-acting elements and by several trans-acting factors, including swi1 (for switch), swi3, swi7, and sap1. swi1 and swi3 were previously shown to function in dictating unidirectional mat1 DNA replication by controlling replication fork movement around the mat1 region and, second, by pausing fork progression around the imprint site. With biochemical studies, we investigated whether the trans-acting factors function indirectly or directly by binding to the mat1 cis-acting sequences. First, we report the identification and DNA sequence of the swi3 gene. swi3 is not essential for viability, and, like the other factors, it exerts a stimulatory effect on imprinting. Second, we showed that only Swi1p and Swi3p interact to form a multiprotein complex and that complex formation did not require their binding to a DNA region defined by the smt-0 mutation. Third, we found that the Swi1p-Swi3p complex physically binds to a region around the imprint site where pausing of replication occurs. Fourth, the protein complex also interacted with the mat1-proximal polar terminator of replication (RTS1). These results suggest that the stimulatory effect of swi1 and swi3 on switching and imprinting occurs through interaction of the Swi1p-Swi3p complex with the mat1 regions.