Chemical inactivation of Pat1: a novel approach to synchronize meiosis.

Progress in the characterization of meio-sis requires the development of tools that improve synchrony, thus facilitating biochemical and cytological analysis. in the fission yeast Schizosaccharomyces pombe, meiosis occurs under starvation conditions in diploid cells (azygotic meiosis) or in zygotes formed after conjugation between haploids of opposite mating type (zygotic meiosis). A key step in the induction of fission yeast meiosis is the inactivation of the Pat1 protein kinase. Pat1 phosphorylates and inhibits the master regulator of meiosis Mei2 and the transcription factor ste11, preventing meiosis in haploid cells and/or in the presence of nutrients. ste11 levels increase under nitrogen starvation, leading to the transcription of its targets, including mei2 and the mating type genes. The products of the mating type genes form a heterodimer that activates the expression of the Pat1 inhibi-tor Mei3. As a consequence, only in diploid cells or zygotes, which harbor both mating type genes, and under nitrogen starvation is Mei3 produced and Pat1 inhibited, leading to Mei2 activation and induction of meiosis. Taking advantage of this signaling pathway, a thermosensitive mutant of pat1 + , pat1–114, has been widely used to synchronize meio-sis independently of ploidy and nutritional signals. in pat1–114 meiosis, haploid cells or diploids homozygous at the mating-type locus are arrested in G 1 by nitrogen starvation, and then Pat1 is inactivated by a temperature shift to 34°C. in the following hours, meiotic events (premeiotic s phase, recombination, meio-sis i, meiosis ii and sporulation) proceed in a very synchronous manner. 3 This methodology has allowed uncovering important features of meiosis that would have otherwise remained unnoticed. synchronous meiosis experiments have facilitated the biochemical analysis of proteins and its modifications, studies on RnA expression and splicing and cytological characterization of fission yeast meiosis. However, meiosis induced in this way is not entirely physiological and, in some aspects, differs from wild type meiosis. Previous work has shown that in pat1–114 meioses, centromere positioning is aberrant; recombination rates are lower; a high proportion of cells show equational instead of reductional meiosis i, and spore viability is reduced. 3,4 Two main concerns arise from pat1–114 meiosis: (1) the absence of pheromone signaling, which can be overcome with the ectopic expression of Mat-Pc or the addition of pheromones, 4,5 (2) the use of high temperature for the inactiva-tion of Pat1, which might be affecting negatively meiosis itself. Two studies published in a previous issue of Cell Cycle propose a …