Spliceosomal introns are non-coding DNA sequences interspersed between exons in eukaryotic genes. Most of them are removed post-transcriptionally by RNA splicing and do not encode proteins, but they have been implicated in various regulatory processes, including gene transcription, mRNA nuclear export, and the generation of protein diversity. The essentiality of spliceosomal introns for fundamental cellular life remains an open question.

In a study published in Cell, a team led by Prof. ZHOU Jinqiu from the Center for Excellence in Molecular Cell Science (Shanghai Institute of Biochemistry and Cell Biology) of the Chinese Academy of Sciences (CAS), constructed a yeast strain named SYNE27^α in which all 300 known spliceosomal introns are eliminated. The viability of this intron‑free strain demonstrates that spliceosomal introns are not essential for yeast survival.

To overcome the challenge of deleting 300 introns sequentially, researchers devised a strategy combining parallel intron deletions in haploids of opposite mating types, followed by site‑specific chromosomal recombination in diploids and meiotic segregation to recover haploid progeny carrying all desired deletions. This strategy reduced the deletion time and whole‑genome sequencing confirmed the precise removal of all 300 introns across 288 genes in SYNE27^α.

Researchers found that the SYNE27^α strain remained stable and could be continuously passaged under standard laboratory conditions, although the cumulative loss of all introns resulted in a slow‑growth phenotype. Importantly, heterozygous diploid cells (SY14/SYNE27^α) which carry a single copy of the intron‑free genome grew normally, indicating that fitness defects associated with intron loss are recessive.

Moreover, researchers showed that deleting genes encoding core spliceosomal components, including splicing factors LUC7, YHC1, PRP8, PRP9, and PRP19, as well as five snRNAs, did not compromise the viability of SYNE27^α. This provide compelling evidence that the primary and essential function of the spliceosome is to process spliceosomal introns rather than to carry out other proposed non‑splicing roles such as transcription‑coupled splicing or nucleocytoplasmic transport.

The SYNE27^α strain represents the first eukaryotic model entirely without spliceosomal introns. This study is a step toward genome streamlining and the construction of minimal eukaryotic genomes. It provides a platform for studying the molecular mechanisms of pre‑mRNA and snoRNA splicing, the evolution of introns, and the principles underlying eukaryotic gene expression regulation.

This work was built upon a single‑chromosome yeast strain developed in 2018 by Prof. QIN Zhongjun's team at the Center for Excellence in Molecular Plant Science of CAS.