Fig 8. Production of TviA is a two-checkpoint system.

Although EnvZ/OmpR induce tviA transcription under low osmolarity conditions, production of the TviA protein only occurs in low salt environments with high temperature due to thermoregulation of tviA translation by the fourU RNAT. This fine-tuning of TviA expression enables S. Typhi to precisely regulate virulence factor expression (i.e., Vi capsule, flagellin, T3SS-1) for optimal evasion of host innate immune responses.