Most fungal cell walls are constructed with significant amounts of chitin, a linear polysaccharide that contributes mechanical resistance to the structure. In the yeast Saccharomyces cerevisiae, chitin is synthesized by three different isozymes, each of which has a separate cellular function. In this yeast, the most important role of chitin is in cytokinesis, when a thin primary septum is synthesized by chitin synthase II to separate mother and daughter cells. If no primary septum can be formed, an irregular remedial septum is synthesized, a process that relies on chitin synthase III. It was found that, with osmotic stabilization, S. cerevisiae tolerates a loss of all chitin synthase activities. Chitin-deficient mutants display a cytokinesis defect which leads to the formation of cell chains with incompletely separated cytoplasms. In these mutants septa are formed rarely. The few septa found are bulky structures which contain inclusions of cytoplasm. Nuclear division proceeds under these conditions, demonstrating that there is no cell cycle arrest triggered by a failure to form a septum between mother and daughter cell. A genetic suppressor arises quickly in chitin-deficient mutants, giving rise to the synthesis of chitin-free remedial septa. The suppressed chitin-free mutants grow well without osmotic stabilization and display hyper-resistance against the chitin-synthase inhibitor polyoxin D.
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