Four chromo-domain proteins of Schizosaccharomyces pombe differentially repress transcription at various chromosomal locations.

Transcription is repressed in regions of the fission yeast genome close to centromeres, telomeres, or the silent mating-type cassettes mat2-P and mat3-M. The repression involves the chromo-domain proteins Swi6 and Clr4. We report that two other chromo-domain proteins, Chp1 and Chp2, are also important for these position effects. Chp1 showed a specificity for centromeric regions. Its essentiality for the transcriptional repression of centromeric markers correlates with its importance for chromosome stability. Chp2 appeared more pleiotropic. Its effects on centromeric silencing were less pronounced than those of Chp1, and it participated in telomeric position effects and transcriptional silencing in the mating-type region. We also found that PolII-transcribed genes were repressed when placed in one of the Schizosaccharomyces pombe rDNA clusters, a situation analogous to that in the budding yeast Saccharomyces cerevisiae. Chp2, Swi6, Clr4, and, to a lesser extent, Chp1 participated in that repression.