Abstract #81

Sense/antisense transcription of the IME4 locus controls a morphogenetic switch in S. cerevisiae. Cintia Hongay, Paula Grisafi, Gerald Fink. Fink Laboratory, Whitehead Institute, Cambridge, MA.
   S. cerevisiae’s IME4 encodes a putative RNA methyltransferase required for sporulation of Mat a/a diploids [1, 2]. We have discovered that an antisense transcript complementary to IME4 (sense) is present in haploid cells, of both a and a mating types, as well as in Mat a/a and Mat a/a diploids, but absent in Mat a/a diploid cells. The cDNA cloning and sequencing of the sense transcript obtained from Mat a/a diploid cells and antisense transcript from haploid cells established the 5' and 3' ends of these transcripts and shows that the antisense transcript overlaps the region encoding the entire sense transcript. Furthermore, antisense IME4 transcription is repressed by binding of the haploid-specific a1/a2 repressor to a conserved binding site found downstream of the IME4 ORF stop. Our data is consistent with mutually exclusive transcription of either sense or antisense according to cell-type. The mutually exclusive transcription at IME4 provides a cell-type specific switch and strongly correlates with the display of either diploid or haploid-like phenotypes. Mat a/a diploids that artificially express antisense IME4 do not express IME4 sense, are unable to sporulate, and show an enhanced agar-adhesion phenotype comparable to that of true haploid cells. Conversely, haploid cells that artificially express IME4 sense cannot express antisense IME4 and have reduced agar-adhesion phenotype. Thus, cell-type dependent sense/antisense transcription of the IME4 locus constitutes a novel cis-acting regulatory mechanism of gene expression involving antisense transcription in S. cerevisiae. This work is being supported by Ruth L. Kirschstein NRSA fellowship to C.H. 1. Shah, J.C. and M.J. Clancy, IME4, a gene that mediates MAT and nutritional control of meiosis in Saccharomyces cerevisiae. Mol Cell Biol, 1992. 12(3): p. 1078-86. 2. Clancy, M.J., et al., Induction of sporulation in Saccharomyces cerevisiae leads to the formation of N6-methyladenosine in mRNA: a potential mechanism for the activity of the IME4 gene. Nucleic Acids Res, 2002. 30(20): p. 4509-18.

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