Histone mRNA is subject to 3´ uridylation and re-adenylation in Aspergillus nidulans

The role of post-transcriptional RNA modification is of growing interest. One example is the addition of non-templated uridine residues to the 3´ end of transcripts. In mammalian systems uridylation is integral to cell cycle control of histone mRNA levels. This regulatory mechanism is dependent on the nonsense mediated decay (NMD) component, Upf1, which promotes histone mRNA uridylation and degradation in response to the arrest of DNA synthesis. We have identified a similar system in Aspergillus nidulans, where Upf1 is required for the regulation of histone mRNA levels. However, other NMD components are also implicated, distinguishing it from the mammalian system. As in human cells, 3´ uridylation of histone mRNA is induced upon replication arrest. Disruption of this 3´ tagging has a significant but limited effect on histone transcript regulation, consistent with multiple mechanisms acting to regulate mRNA levels. Interestingly, 3´ end degraded transcripts are also subject to re-adenylation. Both mRNA pyrimidine tagging and re-adenylation are dependent on the same terminal-nucleotidyltransferases, CutA and CutB, and we show this is consistent with the in vitro activities of both enzymes. Based on these data we argue that mRNA 3´ tagging has diverse and distinct roles associated with transcript degradation, functionality and regulation.