Hysteresis curves of nac protein with respect to ammonia concentration in the wild-type and the GlnK mutants. The mutant lacking GlnK (), the wild type: transcription rate for GlnK = 0.09 min^-1 (), the mutant overexpressing GlnK: transcription rate for GlnK = 0.45 min^-1 (—), and the mutant that greatly overexpresses GlnK: transcription rate for GlnK = 0.9 min^-1 (-------).