Expression of Cre-recombinase in Tetrahymena. (A) Expression of HA-Cre1p in the CRE556 strain is induced by the presence of cadmium ions. B2086 (wild-type) and CRE556 cells were cultured in the nutrient-rich 1× SPP medium (log) or in 10 mM Tris (pH 7.5) (starved) and were treated with (+) or without (-) CdCl2. For log and starved cells, 1 μg/mL and 50 ng/mL CdCl2 were used, respectively. HA-Cre1p was detected by western blotting using an anti-HA antibody. For the loading control, the membrane was stripped using a 2-mercaptoethanol- and SDS-containing buffer and re-probed with antibody against α-tubulin. (B) HA-Cre1p localizes to the macronucleus in Tetrahymena. CRE556 was mated with a wild-type strain and HA-Cre1p expression was induced at 3.5 hr post-mixing (hpm) by adding 50 ng/mL CdCl2. Cells were fixed at 2 hpm (before induction) or at 5 hpm (1.5 hr after induction) and HA-Cre1p was localized using an anti-HA antibody. DNA was counter-stained by DAPI. In each picture, a non-mating (starved) cell (circled with yellow line) and a pair of mating cells (circled with magenta line) are shown. The macro- and micronuclei are marked with "a" and "i", respectively. (C) Expression of HA-Cre1p suppresses growth of Tetrahymena. B2086 (wild-type) or CRE556 were diluted to 5,000 cells/mL with 1× SPP medium with or without 1 μg/mL CdCl2. At indicated time after dilution, cells were counted to monitor cell growth.