lac repressor acts by modifying the initial transcribing complex so that it cannot leave the promoter.

RNA polymerase engaged in the joint complex with the lac repressor at the lac UV5 promoter cannot escape into elongation but generates abortive RNA oligomers. The joint complex actively transcribes a few initial base pairs in a reaction unusually sensitive to a decrease in the substrate concentration. The joint complex, however, fails to traverse a point in the initial transcribed sequence that normally requires a high concentration of the elongating substrate. Thus, the repressor acts by augmenting a natural high "kNTP" site (pause site) embedded in the promoter. A lethal RNA polymerase mutation that mimics the effect of the repressor leads to an analogous block of promoter clearance and shortened abortive product pattern on several promoters, reflecting the widespread occurrence of high kNTP sites in promoters.