Splicing and dicing with a SERRATE d edge

The maturation and quality control of mRNA in eukaryotes is a tightly regulated, multistep process that begins on nascent transcripts. A 7-methyl guanosine (7MeG) cap structure is added to the 5′ end of pre-mRNA as it emerges from the exit channel of RNA Polymerase II. The multifunctional nuclear cap-binding complex (CBC), consisting of two protein subunits (CBP80 and CBP20), assembles at the pre-mRNA cap early during transcript formation and helps recruit the splicesome machinery to the cap-proximal intron (1, 2). Termination of transcription involves cleavage and polyadenylation at the 3′ end, and the mature mRNA is retained in the nucleus or exported to the cytoplasm. In either case, the mRNA undergoes a pioneer round of translation and surveillance by the nonsense-mediated mRNA decay (NMD) pathway to eliminate defective or misspliced transcripts. Although the CBC localizes primarily to the nucleus, it remains associated with mRNAs during export to the cytoplasm and during the pioneer round of translation and mRNA surveillance. After the first round of translation, the CBC is replaced by the eukaryotic initiation complex eIF4F, and the mRNA steady-state translation initiation complex is formed (3). But not all RNA Pol II transcripts are predestined for translation. Primary transcripts for microRNA (pri-miRNA) are retained in the nucleus, where they are processed into ≈21- to 22-nt miRNA that generally function as posttranscriptional regulators of mRNA expression. Although it is known that pri-miRNA transcripts form by RNA Pol II transcription, the extent to which pri-miRNA and pre-mRNA transcripts share common processing components is far from settled. In this issue of PNAS, Laubinger et al. (4) identify an important relationship between mRNA maturation and miRNA primary transcript processing in plants.