Schizosaccharomyces cells divide by medial septation, followed by enzymatic degradation of parts of the septum (septum cleavage) to allow the sister cells to separate from each other. In a previous study we found that the cell separation mutant sep9-307 was defective in a gene that encodes a protein highly similar in sequence to the Saccharomyces cerevisiae protein Spt8, a subunit of the SAGA complex. Here, we show that the sep9-307 mutation causes a frameshift in translation. The deletion of sep9(+) is not lethal but abolishes normal septum cleavage by reducing the activity of ace2(+), a gene coding for a transcription factor of numerous genes producing proteins for septum cleavage. Indirect evidence indicates that Sep9 might also act directly in the transcription of certain target genes (e.g. eng1(+)) of this regulator. sep9-307 is synthetically lethal with mutations in the cell separation genes sep11/med18(+) and sep15/med8(+), which encode subunits of the general transcription factor mediator. Heterologous expression of SPT8 and the putative Schizosaccharomyces japonicus sep9(+) orthologue cannot substitute for sep9(+). Both Spt8 and the fission yeast proteins have highly acidic (74-76 amino-acid long) N-terminal regions with no sequence conservation.