β-galactosidase assay

The assay was carried out essentially as originally described by Miller [53]. All samples were incubated at 28°C and the reaction was started by addition of 200 μl 4 mg/ml ONPG (Sigma-Aldrich #N1127). Reactions that had turned yellow upon visual inspection were stopped by mixing with 500 μl 1 M Na₂CO₃. The samples were spun down and the absorption at 420 nm of the supernatant was measured. The β-galactosidase activity was calculated as (1000 • A₄₂₀) / (TP • t), where ‘TP’ is the total protein concentration in relative units and ‘t’ is the duration of the reaction in minutes.