Cell strains and culture conditions
Tetrahymena thermophila strains used in this work are listed in Table S1 with culture conditions as previously described (Gorovsky et al., 1975). Cells were grown in SPP medium (2% proteose peptone (GIBCO, 211684), 0.1% yeast extract (BD, 212750), 0.2% dextrose (ACROS, 41095-5000) and 0.003% EDTA, ferric-sodium salt) supplemented with 100 μg ml-1 NormocinTM (InvivoGen), an antimicrobial reagent. Cells were grown at 30°C with agitation at 100 rpm. Cell densities were measured using a spectrophotometer (Thermo Spectronic Unicam), where OD550 = 1 corresponds to ~ 1 ×  106 cells ml-1 (David L. Spector, 1998) or a Z1 Coulter Counter (Beckman Coulter Inc., Indianapolis, Indiana). Cultures were generally used during the log phase of cell growth (cell density 2-4 \(\times\)105 cells ml-1). Reagents were from Sigma-Aldrich unless otherwise noted.