Recent studies of mRNA export factors have provided additional evidence for a mechanistic link between mRNA 3¢-end formation and nuclear export. Here, we identify Nab2p as a nuclear poly(A)-binding protein required for both poly(A) tail length control and nuclear export of mRNA. Loss of NAB2 expression leads to hyperadenylation and nuclear accumulation of poly(A) + RNA but, in contrast to mRNA export mutants, these defects can be uncoupled in a nab2 mutant strain. Previous studies have implicated the cytoplasmic poly(A) tail-binding protein Pab1p in poly(A) tail length control during polyadenylation. Although cells are viable in the absence of NAB2 expression when PAB1 is overexpressed, Pab1p fails to resolve the nab2D hyperadenylation defect even when Pab1p is tagged with a nuclear localization sequence and targeted to the nucleus. These results indicate that Nab2p is essential for poly(A) tail length control in vivo, and we demonstrate that Nab2p activates polyadenylation, while inhibiting hyperadenylation, in the absence of Pab1p in vitro. We propose that Nab2p provides an important link between the termination of mRNA polyadenylation and nuclear export.
Sister chromatid cohesion is mediated by cohesin, but the process of cohesion establishment during S-phase is still enigmatic. In mammalian cells, cohesin binding to chromatin is dynamic in G1, but becomes stabilized during S-phase. Whether the regulation of cohesin stability is integral to the process of cohesion establishment is unknown. Here, we provide evidence that fission yeast cohesin also displays dynamic behavior. Cohesin association with G1 chromosomes requires continued activity of the cohesin loader Mis4/Ssl3, suggesting that repeated loading cycles maintain cohesin binding. Cohesin instability in G1 depends on wpl1, the fission yeast ortholog of mammalian Wapl, suggestive of a conserved mechanism that controls cohesin stability on chromosomes. wpl1 is nonessential, indicating that a change in wpl1-dependent cohesin dynamics is dispensable for cohesion establishment. Instead, we find that cohesin stability increases at the time of S-phase in a reaction that can be uncoupled from DNA replication. Hence, cohesin stabilization might be a pre-requisite for cohesion establishment rather than its consequence.
S.Dheur and L.T.A.Vo contributed equally to this workEukaryotic RNA polymerase II transcribes precursors of mRNAs and of non-protein-coding RNAs such as snRNAs and snoRNAs. These RNAs have to be processed at their 3¢ ends to be functional. mRNAs are matured by cleavage and polyadenylation that require a well-characterized protein complex. Small RNAs are also subject to 3¢ end cleavage but are not polyadenylated. Here we show that two newly identi®ed proteins, Pti1p and Ref2p, although they were found associated with the pre-mRNA 3¢ end processing complex, are essential for yeast snoRNA 3¢ end maturation. We also provide evidence that Pti1p probably acts by uncoupling cleavage and polyadenylation, and functions in coordination with the Nrd1p-dependent pathway for 3¢ end formation of non-polyadenylated transcripts.
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