There are multiple RNA degradation mechanisms in eukaryotes, key among these is mRNA decapping, which requires the Dcp1-Dcp2 complex. Decapping is involved in various processes including nonsense-mediated decay (NMD), a process by which aberrant transcripts with a premature termination codon are targeted for translational repression and rapid decay. NMD is ubiquitous throughout eukaryotes and the key factors involved are highly conserved, although many differences have evolved. We investigated the role of Aspergillus nidulans decapping factors in NMD and found that they are not required, unlike Saccharomyces cerevisiae. Intriguingly, we also observed that the disruption of one of the decapping factors, Dcp1, leads to an aberrant ribosome profile. Importantly this was not shared by mutations disrupting Dcp2, the catalytic component of the decapping complex. The aberrant profile is associated with the accumulation of a high proportion of 25S rRNA degradation intermediates. We identified the location of three rRNA cleavage sites and show that a mutation targeted to disrupt the catalytic domain of Dcp2 partially suppresses the aberrant profile of Δdcp1 strains. This suggests that in the absence of Dcp1, cleaved ribosomal components accumulate and Dcp2 may be directly involved in mediating these cleavage events. We discuss the implications of this.
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.