The 5′ terminal region of the Schizosaccharomyces pombe mes1 mRNA is crucial for its meiosis-specific splicing

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Abstract
The mes1 + gene of Schizosaccharomyces pombe is required for the second meiotic division. The single 75-nt intron in mes1 is spliced out only in meiotic cells. Here we report a cis-acting element which is responsible for meiosis-specific splicing. Both 5′ and 3′ splice sites of the mes1 intron deviate from the consensus sequence. Point mutations which altered these sites so that they conformed to the consensus, however, did not affect the splicing pattern of mes1. Neither replacement of the mes1 intron with the constitutively spliced intron of the nda3 gene, nor replacement of the 3′ exon with E. coli lacZ changed the splicing pattern. In contrast, deletion of the 5′ terminal 125 nt from the 5′ exon derepressed splicing in vegetative cells, implying that this 5′ terminal sequence, named SRE (mes1 splicing repression element), inhibits splicing of the downstream intron. A potential stem-loop structure in the SRE is predicted. Disruption of this stem structure by mutation abolished the repression of mes1 splicing in vegetative cells. Overexpression of the SRE sequence on a multicopy plasmid also relieved the repression of splicing of the authentic mes1 transcripts. These results suggest that as yet unknown trans-acting factors inhibit splicing of the mes1 transcript in vegetative cells by interacting with the cis-element SRE.

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