Sap1, a protein that binds to sequences required for mating-type switching, is essential for viability in Schizosaccharomyces pombe.

Abstract

The pattern of mating-type switching in cell pedigrees of the fission yeast Schizosaccharomyces pombe is dictated by the inheritance of specific DNA chains at the mating-type locus (mat1). The recombination event essential for switching is initiated by a site-specific double-strand break at mat). The switch-activating protein, Sapl, binds in vitro to a mat] cis-acting site that was shown earlier to be essential for efficient mating-type switching. We isolated the sap) gene by using oligonucleotides corresponding to the amino acid sequence of purified Sapl protein. (12,32). Consequently, a colony grown from a single cell of either P or M mating type contains nearly equal proportions of P and M cells (for reviews, see references 3, 13, 21, and 27). Pedigree analysis of single cells grown under starvation conditions, in which mating and sporulation are induced, revealed that switching is not random but follows several rules. First, the mating-type switching is formally a G2 event, as only one of the two daughters of a switching-competent cell actually switches. Second, two consecutive developmentally asymmetric cell divisions are required to restrict switching to only one cell among four related sister and cousin cells. The first asymmetric division of a switching-incompetent cell produces two unswitched cells that differ in their potential for switching: one is switching competent, while the other, like its parent, is not. The second asymmetric division of the switching competent cell produces one switched cell and one unswitched cell (35). Third, the switching potential is inherited by the sister of the recently switched cell, which can repeat the process of switching again and again, producing a chain of consecutive switching (15,26). The observed pattern of switching is analogous to that of the mammalian stem cell lineage, because the parental cell

Diverse roles of HP1 proteins in heterochromatin assembly and functions in fission yeast

“…2 A). Other notable proteins included Sap1, which has a role in DNA replication and mating-type switching (31,32), telomere associated proteins (such as Ccq1 and Rap1) implicated in heterochromatin assembly (22,33), and components of the microtubule organizing center known as the spindle pole body (SPB) (Fig. 2 A and Fig.…”

Sap1p Binds to Ter1 at the Ribosomal DNA of Schizosaccharomyces pombe and Causes Polar Replication Fork Arrest

“…Sap1p is an essential gene (32,33), thereby preventing analysis of Ter1 activity in sap1-null mutants. Similarly, overexpression of the protein is toxic, apparently causing chromosome fragmentation and rapid cell death (33).…”

“…Loss of Sap1p causes defects in chromosome segregation, whereas overexpression of Sap1p is also toxic, causing pleiotropic effects including chromosome fragmentation and abnormal mitosis. These phenotypes occur after the initiation of DNA replication (32). Although these effects are likely initiated by Sap1p binding to numerous sites throughout the genome, the targets remain elusive as no binding sites apart from SAS1 have yet been identified.…”

“…Forks also pause in a Swi1p-and Swi3p-dependent manner at the site of the imprint (27,29). Imprinting requires in addition the catalytic subunit of DNA polymerase ␣ Swi7p (30), as well as Sap1p, which binds to its cognate site SAS1, located ϳ160 bp from the site of the imprint (31,32). Although necessary for the formation and/or maintenance of the imprint, neither Swi7p nor Sap1p are required for the initial fork pausing event (27,31).…”

“…Although necessary for the formation and/or maintenance of the imprint, neither Swi7p nor Sap1p are required for the initial fork pausing event (27,31). Sap1p is an essential DNA-binding protein and is required for viability independently of its function in mating type switching (32), perhaps because of its chromatin-organizing function (33). Loss of Sap1p causes defects in chromosome segregation, whereas overexpression of Sap1p is also toxic, causing pleiotropic effects including chromosome fragmentation and abnormal mitosis.…”