Gene expression regulation in fission yeast : from splicing to MBF-dependent transcription

Schizosaccharomyces pombe initiates sexual development under nitrogen starvation, changing the transcription patterns of many genes to allow sexual differentiation. MBF transcription factor recognize specific sequences present in genes required for mitotic and meiotic S phase and activates their transcription through binding to their promoters. Genes involved in recombination (rec genes) are also activated by MBF, but in a different and independent wave of transcription. Res2 plays an important role in the transcription of this new wave, as higher levels of this protein are required to initiate recombination. Another form of gene expression regulation takes place through splicing. Spliceosome is in charge of these transesterification reactions and requires a correct recognition of intron-exon boundaries to achieve proper splicing efficiency. Auxiliary factors are essential for a correct splicing of introns with degenerated sequences, as Prp2, being the last nucleotide of the upstream exon what determines the dependency of an intron on Prp2.Schizosaccharomyces pombe inicia el proceso de meiosis bajo la restricción de nutrientes, cambiando los patrones de transcripción de muchos genes. El factor de transcripción MBF reconoce secuencias específicas presentes en genes necesarios para la fase de replicación del DNA mitótica y meiótica, y activa su transcripción a través de la unión a sus promotores. Los genes encargados de la recombinación meiótica también son activados por MBF, pero en una ola de transcripción diferente e independiente a la de la fase de replicación. Res2 juega un papel importante en la transcripción de esta nueva ola, puesto que se requieren niveles altos de esta proteína para iniciar la recombinación. Otra forma de regulación de la expresión génica tiene lugar a través del splicing. Una adecuada definición de los bordes intrón-exón es necesaria para que el espliceosoma se forme correctamente. Los factores de splicing tienen un papel fundamental en el reconocimiento de los sitios de splicing con secuencias no-consenso, como Prp2, siendo el último nucleótido del exón upstream lo que determina la dependencia de Prp2 de un determinado intrón

A systematic screen identifies Saf5 as a link between splicing and transcription in fission yeast

Splicing is an important step of gene expression regulation in eukaryotes, as there are many mRNA precursors that can be alternatively spliced in different tissues, at different cell cycle phases or under different external stimuli. We have developed several integrated fluorescence-based in vivo splicing reporter constructs that allow the quantification of fission yeast splicing in vivo on intact cells, and we have compared their splicing efficiency in a wild type strain and in a prp2-1 (U2AF65) genetic background, showing a clear dependency between Prp2 and a consensus signal at 5' splicing site (5'SS). To isolate novel genes involved in regulated splicing, we have crossed the reporter showing more intron retention with the Schizosaccharomyces pombe knock out collection. Among the candidate genes involved in the regulation of splicing, we have detected strong splicing defects in two of the mutants -Δcwf12, a member of the NineTeen Complex (NTC) and Δsaf5, a methylosome subunit that acts together with the survival motor neuron (SMN) complex in small nuclear ribonucleoproteins (snRNP) biogenesis. We have identified that strains with mutations in cwf12 have inefficient splicing, mainly when the 5'SS differs from the consensus. However, although Δsaf5 cells also have some dependency on 5'SS sequence, we noticed that when one intron of a given pre-mRNA was affected, the rest of the introns of the same pre-mRNA had high probabilities of being also affected. This observation points Saf5 as a link between transcription rate and splicing.This work was supported by grants BFU2018-PGC2018-097248-B-I00 and PID2022-136449NB-I00 funded by MICIU/AEI/10.13039/501100011033 and ERDF/EU to JA and by Unidad de Excelencia Maria de Maeztu (CEX2018-000792-M) to JA and EH. EH is a recipient of an ICREA Academia Award (Generalitat de Catalunya). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript