RAN1 + controls the transition from mitotic division to meiosis in fission yeast

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Abstract
Summary We have investigated the genetic and physiological control of meiosis in fission yeast. Nutritionally depleted h +/h − diploid cells become irreversibly commited to meiosis immediately prior to the initiation of premeiotic S phase. Premeiotic DNA synthesis requires matP +, matM +, mei2 + and mei3 + but not the mitotic cell cycle control gene, cdc2 +, ran1 + is an essential gene, loss of which provokes sexual conjugation, premeiotic DNA synthesis, pseudo-meiosis and the sporulation of haploid cells. Our experiments suggest that sexual differentiation is achieved physiologically by the inhibition of ran1 + activity in a two-step process. In the first step, partial inhibition of ran1 + in starved haploid cells, leads to cell cycle arrest in G1 followed by sexual conjugation. In the second step, a pathway requiring the matP +, matM + and mei3 + genes of the newly-formed zygote, further inhibits ran1 + and thereby commits the cell to meiosis. mei2 + is required for meiotic commitment after full inhibition of ran1 +. ran1 + is normally essential for vegetative cell reproduction but is inessential in cells which have abnormally high levels of cAMP-dependent protein kinase. We propose that the ran1 + gene encodes a highly controlled protein kinase which shares key substrates with cAMP-dependent protein kinase.