An Antisense RNA-mediated Mechanism Eliminates a Meiosis-specific Copper-regulated Transcript in Mitotic Cells*

Abstract

Sense and antisense transcripts produced from convergent gene pairs could interfere with the expression of either partner gene. In Schizosaccharomyces pombe, we found that the iss1⁺ gene produces two transcript isoforms, including a long antisense mRNA that is complementary to the meiotic cum1⁺ sense transcript, inhibiting cum1⁺ expression in vegetative cells. Inhibition of cum1⁺ transcription was not at the level of its initiation because fusion of the cum1⁺ promoter to the lacZ gene showed that activation of the reporter gene occurs in response to low copper conditions. Further analysis showed that the transcription factor Cuf1 and conserved copper-signaling elements (CuSEs) are required for induction of cum1⁺-lacZ transcription under copper deficiency. Insertion of a multipartite polyadenylation signal immediately downstream of iss1⁺ led to the exclusive production of a shorter iss1⁺ mRNA isoform, thereby allowing accumulation of cum1⁺ sense mRNA in copper-limited vegetative cells. This finding suggested that the long iss1⁺ antisense mRNA could pair with cum1⁺ sense mRNA, thereby producing double-stranded RNA molecules that could induce RNAi. We consistently found that mutant strains for RNAi (dcr1Δ, ago1Δ, rdp1Δ, and clr4Δ) are defective in selectively eliminating cum1⁺ sense transcript in the G₁ phase of the cell cycle. Taken together, these results describe the first example of a copper-regulated meiotic gene repressed by an antisense transcription mechanism in vegetative cells.