TABLE 1.

pH regulates white-to-opaque switching in strains with different backgrounds^a

Strain	Culture condition	White-to-opaque switching frequency (%) at pH:
		5.0	6.0	7.0	7.5	8.0
WO-1 (FC4, α/α)	Air	29.1 ± 4.8*	2.9 ± 0.9	2.3 ± 0.2	3.0 ± 0.5	3.5 ± 0.2
	5% CO2	100.0	100.0*	69.4 ± 7.5	49.5 ± 13.4	66.8 ± 3.4
SN250α (α/α)	Air	19.7 ± 8.4	15.9 ± 2.2	15.3 ± 8.4	12.8 ± 7.6	NA
	5% CO2	90.3 ± 7.8	82.4 ± 25.7*	27.7 ± 21.5	18.3 ± 2.4	NA
12C (a/a)	Air	<0.5	<1.0	<0.5	<0.5	<0.4
	5% CO2	22.3 ± 5.9*	13.0 ± 12.2*	<1.0	<1.5	1.0 ± 1.8
P37005 (a/a)	Air	<0.3	<1.0	<0.4	<0.5	<1.0
	5% CO2	2.7 ± 0.6*	1.3 ± 0.5	<0.5	<0.4	<0.4

^aTwo α/α strains (strain FC4, a stock of WO-1, and strain SN250α) and two a/a strains were used. White cells were plated onto Lee's glucose medium plates and incubated in air or 5% CO₂ at 25°C for 7 days. Assays for the quantitation of the white-to-opaque switching frequencies were performed. <, no opaque colonies or colonies with opaque sectors were observed. *, the switching rate is significantly (Student's t test, P < 0.05) higher than that under the condition at the next higher pH. For example, the switching rate of strain WO-1 at pH 5.0 (29.1 ± 4.8) was significantly higher than that at pH 6.0 (2.9 ± 0.9). NA, not available.