When pN recognizes the "nut" site, it forms a persistent antitermination complex in cooperation with a number of E. coli host proteins. These include three host Nus proteins, NusA, B, and C. NusA is an interesting protein. By itself in E. coli, it is part of the transcription termination system. However, when co-opted by N, it participates in antitermination. The complex must act on RNA polymerase to ensure that the enzyme can no longer respond to the terminator. The variable locations of the "nut" sites indicate that this event is linked neither to initiation nor to termination, but can occur to RNA polymerase as it elongates the RNA chain past the nut site. Phages that are related to lambda have different N genes and different antitermination specificities. The region on the phage genome in which the "nut" sites lie has a different sequence in each of these phages, and each phage must therefore have characteristic "nut" sites recognized specifically by its own pN. Each of these pN products must have the same general ability to interact with the transcription apparatus in an antitermination capacity, but each product also has a different specificity for the sequence of DNA that activates the mechanism.