Figure 4.

Outline of the regulation of meiotic initiation by Pat1 kinase and its substrate Mei2. During vegetative growth, Pat1 kinase is functional and inactivates Mei2 by phosphorylation. In diploid cells exposed to nutritional starvation, an inhibitor of Pat1 kinase, Mei3, is produced and blocks the kinase activity of Pat1. This, together with enhanced transcription of the mei2 gene under starved conditions, results in the accumulation of the unphosphorylated form of Mei2, which then turns on meiosis.