Fig. 4.

Wor1 induces its own transcription and bistable expression of WOR1 in single cells. (A) Expression of Wor1 from the inducible MAL2 promoter promotes the expression of WOR1 from its own promoter and the transition to opaque phase. MTLa cells (JYC5) transformed with the pMAL2-WOR1-HA construct from a fresh white colony were grown to logarithmic phase in SD, washed five times, and resuspended into SC/maltose at 25°C. Cells were harvested at the indicated times for Northern blot analysis and plated to determine the percentages of opaque colony formation. The percentage of opaque colonies on SD plates with 5 μg/ml phloxine B is shown for each time point. MTLa wor1 transformed with the pMAL2-WOR1-HA construct was included to show the WOR1 transcribed from the MAL2 promoter. (B) Expression of WOR1 shows an on-and-off pattern in single cells. Differential interference contrast microscopy (DIC) and FITC images were taken with cells of MTLa pWOR1-GFP (JYC1 plus pWOR1-GFP) collected from a white colony, an opaque colony, and a sector with both white and opaque cells. GFP expression is under the control of the WOR1 promoter in this strain.