Fission yeast heterochromatin is formed at centromeres, telomeres, and in the 
mating-type region where it mediates the transcriptional silencing of the mat2-P 
and mat3-M donor loci and the directionality of mating-type switching. We 
conducted a genetic screen for directionality mutants. This screen revealed the 
essential role of two previously uncharacterized factors, Clr7 and Clr8, in 
heterochromatin formation. Clr7 and Clr8 are required for localization of the 
Swi6 chromodomain protein and for histone H3 lysine 9 methylation, thereby 
influencing not only mating-type switching but also transcriptional silencing in 
all previously characterized heterochromatic regions, chromosome segregation, 
and meiotic recombination in the mating-type region. We present evidence for 
physical interactions between Clr7 and the mating-type region and between Clr7 
and the S. pombe cullin Pcu4, indicating that a complex containing these 
proteins mediates an early step in heterochromatin formation and implying a role 
for ubiquitination at this early stage prior to the action of the Clr4 histone 
methyl-transferase. Like Clr7 and Clr8, Pcu4 is required for histone H3 lysine 9 
methylation, and bidirectional centromeric transcripts that are normally 
processed into siRNA by the RNAi machinery in wild-type cells are easily 
detected in cells lacking Clr7, Clr8, or Pcu4. Another physical interaction, 
between the nucleoporin Nup189 and Clr8, suggests that Clr8 might be involved in 
tethering heterochromatic regions to the nuclear envelope by association with 
the nuclear-pore complex.