Fission yeast cell morphogenesis: identification of new genes and analysis of their role during the cell cycle.

To identify new genes involved in the control of cell morphogenesis in the fission yeast Schizosaccharomyces pombe we have visually screened for temperature-sensitive mutants that show defects in cell morphology. We have isolated and characterized 64 mutants defining 19 independent genes, 10 of which have not been previously described. One class of mutants, defining 12 orb genes, become round and show a complete loss of cell polarity. A second class of mutants exhibits branched or bent morphologies. These mutants show defects in either selection of the growth site, defining two tea genes, or in the maintenance of growth direction, defining five ban genes. Immunofluorescence analysis of these morphological mutants shows defects in the organization of the microtubule and actin cytoskeleton. These defects include shortened, bundled, and asymmetrically localized microtubules and enlarged and mislocalized actin patches. Analysis of the mutant phenotypes has allowed us to order the genes into four groups according to their function during the cell cycle: genes required for the maintenance of cell polarity throughout the cell cycle; genes necessary only for the reestablishment of cell polarity after mitosis and not for maintaining cell polarity once it is established; genes essential for the transition from monopolar to bipolar growth and genes that severe as 'polarity markers'.