Fig. 2. btn1 cells have defects in septa and the cell wall that are worse at 37°C than at 25°C. (A) btn1 cells at 25°C have thicker septa compared with wild-type cells. (i) High-pressure freezing EM of wild-type 972 and btn1 cells, showing a cross-section through the outer third of the division septum (arrowheads indicate `white' primary septum, arrows show outer secondary septum). Scale bars: 100 nm. (ii) Graph of septum thickness in nm (wild-type 972, n=5; btn1, n=6; ^**P<0.01). (B) btn1 cells at 25°C have thicker cell walls compared with wild-type cells. High-pressure freezing EM of the cell-wall cross-section of wild-type 972 and btn1 cells at 25°C. Scale bars: 100 nm. (C) btn1 cells have thicker septa and cell walls at 37°C compared with wild-type cells. (i) High-pressure freezing EM representative of septated wild-type 972 and btn1 cells after 18 hours at 37°C (arrowheads indicate primary septa; arrows show the secondary septum; L, lipid droplets; V, vacuole; N, nucleus). Scale bars: 2 µm. (ii) Graph of septum thickness in nm (wild-type 972, n=6; btn1, n=6; ^***P<0.001). (iii) High-pressure freezing EM representative of interphase wild-type 972 and btn1 cells after 18 hours at 37°C (arrow indicate excess cell-wall deposition). (D) Growth of btn1 cells is rescued by sorbitol. (i) Wild-type 972 and btn1 cells grown on YES agar or YES + 1 M sorbitol agar at 37°C for 3 days. (ii) Phase-contrast images of the indicated cells grown in YES + 1 M sorbitol for 18 hours at 37°C. Scale bars: 10 µm. (E) The septation index of btn1 cells, which is increased at 25°C compared with wild-type cells, is restored to near wild-type levels by growth in sorbitol. Graph of septation indices of wild-type 972 and btn1 cells grown in the indicated medium. n=300 (three replicates).