Figure 4.

Model of Grc3 kinase activity involvement in the process of transcription termination by Pol I. Rnt1 co-transcriptionally cleaves the transcript RNA at a stem loop structure in the 3′-ETS. The phosphorylation status of the Rnt1 cleavage 3′-product is controlled by the equilibrium between a putative phosphatase activity and the RNA kinase Grc3. The phosphorylated 5′-end (top) is recognized by the 5′–3′ exonuclease Rat1 that ‘torpedoes' Pol I and thus promotes transcription termination. When Grc3 is absent or inactive, the equilibrium is shifted and the 3′-transcripts present a 5′-hydroxyl end (bottom). This is suboptimal substrate for the ‘torpedo' Rat1. As a consequence, the 3′-transcripts are stabilized and Pol I termination is impaired. ETS, external transcribed sequence; Pol I, RNA polymerase I; rRNA, ribosomal RNA.