Discovery and Functional Analysis of the Single-Celled Yeast NADPH Oxidase, Yno1

In this chapter, we describe the discovery of the NADPH oxidase gene and protein of the single-celled yeast Saccharomyces cerevisiae, Yno1. This enzyme was characterized with respect to mechanism of action, subcellular location, regulation of gene expression, and physiological function. Yno1 is not involved in defense and is not highly expressed in vegetatively growing cells. However, it is expressed in diverse stress situations. The signaling substance produced by Yno1 in conjunction with the superoxide dismutase Sod1, hydrogen peroxide, consequently leads through a change in the expression of target genes to the modulation of an adaptive cellular response. An example is the formation of pseudohyphae enabling invasive growth of the yeast cells, which is believed to aid in the utilization of new nutrients. The major role of Yno1 is in the switch of the mode of growth from vegetative budding to the formation of pseudohyphae, which are elongated chains of cells. Further examples that are described in this chapter are the response to osmotic stress and mating. All these pathways have in common that they exit the regular cell cycle and are associated with in parts enormous changes in cell morphology. This is accomplished involving a change in the structure of the actin cytoskeleton. Yno1 was shown to directly modulate the actin cytoskeleton.