In the "trp" operon, tryptophan binds to the repressor protein effectively blocking gene transcription. The image to the right demonstrates regulation by a repressor in the lac operon. The absence of the repressor allows the transcription of the "lac" operon to proceed. The "tra" operon includes genes required for conjugation and plasmid transfer. This allows RNA polymerase to transcribe the operon. In this situation, repression is that of RNA polymerase transcribing the genes in the operon. The bacterial luciferin-luciferase system is encoded by a set of genes labelled the "lux" operon. So it is a negative inducible operon induced by presence of lactose or allolactose. RNA polymerase is free to continue transcribing the entire operon. Attenuation is a second mechanism of negative feedback in the trp operon. A classic example of positive feedback is the lac operon in "E. coli". The latter is produced by the "lac" operon in "Escherichia coli". An engineered minimal 10-gene operon that incorporates these additional essential genes has been constructed. TraJ is a transcription factor that upregulates the "tra" operon. Attenuation is a second mechanism of negative feedback in the "trp" operon.