In yeast, the pseudohyphal phenotype induced by isoamyl alcohol results from the operation of the morphogenesis checkpoint.

Isoamyl alcohol (IAA) induces a phenotype that resembles pseudohyphae in the budding yeast Saccharomyces cerevisiae. We show here that IAA causes the rapid formation of linear chains of anucleate buds, each of which is accompanied by the formation of a septin ring at its neck. This process requires the activity of Swe1 and Slt2 (Mpk1). Cdc28 is phosphorylated on tyrosine 19 in a Swe1-dependent manner, while Slt2 becomes activated by dual tyrosine/threonine phosphorylation. Tyrosine 19 phosphorylation of Cdc28 is not dependent on Slt2. However, the defective response in the slt2Delta mutant is rescued by an mih1Delta mutation. The IAA response still occurs in a cell containing a dominant non-phosphorylatable form of Cdc28, but no longer occurs in an mih1Delta slt2Delta mutant containing this form of Cdc28. These observations show that IAA induces the Swe1-dependent morphogenesis checkpoint and so the resulting pseudohyphal phenotype arises in an entirely different way from the formation of pseudohyphae induced by nitrogen-limited growth.