Practice question
Question
Which RNA polymerase transcribes centromeric repeats?
Explanation
Pericentromeric repeats dg and dh in fission yeast Schizosaccharomyces pombe are transcribed during S phase by RNA polymerase II despite heterochromatic state generating noncoding RNAs serving as precursors for RNAi-mediated heterochromatin assembly. This Pol II transcription transient and actively suppressed by Clr4-mediated H3K9 methylation and Swi6 HP1 binding, yet essential to recruit RDRC RNA-dependent polymerase, Dicer Dcr1, and RITS effector complex for dsRNA processing into siRNAs guiding heterochromatin formation. Pol I synthesizes rRNA, Pol III tRNAs, not centromeres. Co-transcriptional processing ensures chromatin assembly coupled to replication, maintaining centromere identity and kinetochore formation necessary for accurate chromosome segregation during subsequent mitosis.
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