An antisense RNA-mediated mechanism eliminates a meiosis-specific copper -regulated transcript in mitotic cells

Eukaryotic cell division undergoes either mitosis or meiosis.  In the case of meiosis, this process allows diploid precursor cells to produce haploid gametes, which are required for sexual reproduction.  Because several specialized proteins are involved in meiosis-specific mechanisms such as meiotic homologous recombination, meiotic nuclear divisions, and forespore morphogenesis, their presence is detrimental to cells proliferating in mitosis.  Cells have therefore developed different mechanisms to prevent or eliminate meiosis-specific transcripts during mitosis.  One mechanism of mRNA elimination involves genes that are organized into convergent pairs.  In this case two genes are in proximity of one another with their transcription orientated one toward the other.  We have recently discovered that Schizosaccharomyces pombe iss1⁺ and cum1⁺ genes are arranged with their transcription orientated convergently.  The iss1⁺ gene produces two transcript isoforms, including a long antisense mRNA that is complementary to the meiotic cum1⁺ sense transcript.  When the production of the long iss1⁺ antisense transcript was disrupted by insertion of a polyadenylation signal immediately downstream of iss1⁺, results showed high levels of cum1⁺ sense mRNA during mitosis, which were copper starvation- and Cuf1-dependent.  This finding suggested that inhibition of expression of cum1⁺ sense mRNA occurs due to the presence of the long iss1⁺ antisense RNA.  In fission yeast, sense/antisense RNA duplexes accumulate in G1 phase of the cell cycle, especially in regions where convergent genes are present.  In G1, transcription of several convergent genes fails to terminate after their proximal cleavage and polyadenylation sites, thereby resulting in a transcriptional read-through that produces long antisense/sense transcripts.  Accumulation of long antisense/sense RNA duplexes activates the RNAi pathway, which leads to gene silencing and heterochromatin formation over convergent gene regions.  To assess involvement of the RNAi machinery in silencing cum1⁺ transcription, we probed cum1⁺ mRNA levels in four mutants that affect the RNAi pathway.  These mutants were dcr1D (dicer), ago1D (argonaute), rdp1D (RNA-dependent RNA polymerase) and clr4D (histone methyltransferase).  Using G1-synchronized wild-type and dcr1D, ago1D, rpd1D, and clr4D mutant cells, we determined that components of the RNAi pathway are involved in antisense-mediated repression of cum1⁺ transcription, especially during the G1 phase of the cell cycle.  In this study, we have uncovered a mechanism by which a copper- and meiosis-specific transcript is selectively inhibited in growing cells, preventing its untimely expression during mitosis.  To our knowledge, this is the first report to show an antisense transcription control of a copper-regulated transcript under physiological low copper conditions.