FIGURE 7.

Model for uridylation of the 3′ end of histone mRNA. The 3′ end of histone mRNA is formed by trimming by 3′hExo. If 3′hExo continues trimming, the length of the 3′ end is restored by the action of TUT7 to add uridines to return them to the proper length. Several cycles of this may occur. When DNA replication is inhibited, Upf1 is recruited to the 3′ end of histone mRNA, the 3′ end of the mRNA is uridylated and then 3′hExo degrades into the stem. This intermediate is uridylated by TUT7 and then subsequently degraded either 5′–3′ or 3′–5′.