Researchers have mapped, in real time, the order in which RNA-binding proteins load onto nascent transcripts at actively transcribing genes, revealing a precise temporal choreography of messenger ribonucleoprotein (mRNP) assembly. The study, published in Cellular & Molecular Biology Letters, used fluorescently tagged RBPs and mRNAs in cell systems that allow the detection of actively transcribing genes, enabling direct observation of factor recruitment as it happens.
To disentangle mRNP assembly from the splicing machinery, the team employed an inducible, intronless long reporter gene. This design allowed them to map the sequential loading of factors onto active transcripts independently of splicing, addressing a long-standing limitation in the field, where gene-specific sequential RBP recruitment for mRNP assembly has been poorly understood.
One of the central findings is a measurable temporal gap between the arrival of factors that bind the two ends of the transcript. Early-recruiting 5′-end factors, such as the cap-binding protein CBP80, appear at the active transcription site well before later-arriving 3′-end factors such as CPSF6. The researchers report that this gap corresponds to an RNA polymerase II elongation rate of approximately 3.8 kb/min, providing a quantitative link between the timing of protein recruitment and the speed at which the polymerase travels along the gene.
The study also captured the sequential recruitment of the m6A RNA modification machinery to nascent transcripts, demonstrating that components responsible for installing this widespread modification arrive in an ordered fashion during transcription rather than after transcript release.
Not all protein complexes behaved alike. Certain components of the TREX complex were robustly recruited to the active site of transcription, whereas subunits of the exon-junction complex (EJC) were notably absent from the transcription site of the intronless reporter genes.
Within the 3′-end assembly pathway, the researchers uncovered a clear hierarchy. The DEAD-box helicase DDX39B arrives at the nascent mRNA prior to the export adaptor ALYREF, establishing an ordered sequence of recruitment among factors involved in preparing transcripts for nuclear export.
The team further tested which factors are required for ALYREF’s cotranscriptional recruitment. They found that this recruitment is critically dependent on the poly(A) binding protein PABP2, but not on CPSF6. This dependency pattern distinguishes between two 3′-end-associated factors and points to PABP2 as a key upstream player in delivering the export adaptor to newly made mRNA.
By tracking these events in living cells, the study links the stepwise loading of RNA-binding proteins to the transcriptional control of gene expression. The findings delineate how capping-associated, modification, export and 3′-end factors are scheduled along the length of a nascent transcript, and how that schedule is shaped by polymerase elongation. The work was carried out by Hila Hamiel-Levi, Si Hanoh, Irit Shoval and Yaron Shav-Tal, and is available as an open-access article published on 28 September 2026.
Subject of Research: Temporal recruitment of RNA-binding proteins to nascent intronless transcripts during mRNP assembly
Article Title: Investigating the temporal recruitment of RNA-binding proteins to nascent intronless transcripts during mRNP formation
Article References: Hamiel-Levi, H., Hanoh, S., Shoval, I., & Shav-Tal, Y. (2026). Investigating the temporal recruitment of RNA-binding proteins to nascent intronless transcripts during mRNP formation. Cellular & Molecular Biology Letters. https://doi.org/10.1186/s11658-026-01029-0
Image Credits: AI Generated
DOI: 10.1186/s11658-026-01029-0
Keywords: RNA-binding proteins, mRNP assembly, transcription, RNA polymerase II, intronless transcripts, TREX complex, m6A, DDX39B, ALYREF, PABP2, CPSF6, gene expression
News Source: Juliet Wilcox. (October 29, 2026). Scientists Watch RNA-Binding Proteins Assemble on Nascent Transcripts in Real Time. Scienmag.
