TLR4 and infection: Interestingly, induction of CHOP expression by C. trachomatis or C. muridarum infection was entirely GCN2 dependent indicating that although infection resulted in PKR activation, GCN2 is the likely eIF2α kinase responsible for the induction of the ISR and is independent of IRE1α, PKR, PERK and TLR4 signalling.