Figure 3, Key Figure: A model for G4C2||G2C4 transcription.

Top panel: Transcription elongation factors DSIF (Spt4/5) and PAF1C (Paf1/ Leo1/ CDC73/ Ctr9/ Rtf1) are recruited to, and are important for, RNAPII-transcription at G4C2-repeats – potentially acting during the elongation phase. Middle panel: in flies, depletion of Spt4, or specific Paf1C components – CDC73, Ctr9, and Rtf1 –alters expression from both inert <30 G4C2 repeats and toxic >30 G4C2 repeats. Bottom panel: by contrast, PAF1C components Paf1 and Leo1 (which form a heterodimer within PAF1C) seem most important for transcription from the toxic >30 G4C2 repeat, as their depletion only impacted G4C2-transcription in disease situations. Data indicate that PAF1C components are upregulated in response to the expression of >30 G4C2 repeats in flies, mice, and patient-derived cells[18]. Further, hSUPT4H1, hPAF1 and hLEO1, but not hCDC73, are upregulated in C9+ patient tissue, potentially the result of an increased requirement for these factors’ as they enhance transcription of the gene carrying an expanded repeat. We hypothesize that transcription of the expanded G4C2 DNA requires PAF1C and RNAPII to interact during elongation through the repeat expansion, supported by data that PAF1C is bound to G4C2-chromatin immediately 3’ of the repeat in C9+ derived patient cells[18]. DSIF may be important for recruiting PAF1C to the G4C2-repeat expansion.