Knockouts of TUT7 and 3′hexo Show That They Cooperate in Histone Mrna Maintenance and Degradation

Metazoan histone mRNAs are the only cellular eukaryotic mRNAs that are not polyadenylated, ending instead in a conserved stem–loop. SLBP is bound to the 3′ end of histone mRNAs and is required for translation of histone mRNA. The expression of histone mRNAs is tightly cell-cycle regulated. A major regulatory step is rapid degradation of histone mRNA at the end of S-phase or when DNA synthesis is inhibited in S-phase. 3′hExo, a 3′ to 5′ exonuclease, binds to the SLBP/SL complex and trims histone mRNA to 3 nt after the stem–loop. Together with a terminal uridyl transferase, 3′hExo maintains the length of the histone mRNA during S-phase. 3′hExo is essential for initiating histone mRNA degradation on polyribosomes, initiating degradation into the 3′ side of the stem–loop. There is extensive uridylation of degradation intermediates in the 3′ side of the stem when histone mRNA is degraded. Here, we knocked out TUT7 and 3′hExo and we show that both modification of histone mRNA during S-phase and degradation of histone mRNA involve the interaction of 3′hExo, and a specific TUTase, TENT3B (TUT7, ZCCHC6). Knockout of 3′hExo prevents the initiation of 3′ to 5′ degradation, stabilizing histone mRNA, whereas knockout of TUT7 prevents uridylation of the mRNA degradation intermediates, slowing the rate of degradation. In synchronized 3′hExo KO cells, histone mRNA degradation is delayed, but the histone mRNA is degraded prior to mitosis by a different pathway.