Regulation Of λ N-Gene Expression

The N gene product of phage λ is a positive regulatory factor for the transcription of other λ genes during phage development. It acts by modifying the RNA polymerase transcription complex. In the absence of N function, RNA polymerase initiates at the early λ promoters, p L and p R,but terminates transcription shortly after initiation at terminators, t L1 and t R1 respectively. The action of N prevents polymerase from terminating at t L1 and t R1 as well as at other more distant terminators in the p L and p R operon (see review by Friedman, 1988). In each of these operons, between the promoter and the first terminator, there is a target sequence which is also required for the N- antitermination process (Friedman et al., 1973; Rosenberg et al, 1978). These targets, nutL and nutR, have been defined genetically by mutations to block antitermination in the respective operons (Salstrom and Szybalski, 1978; Olson, et al., 1984). These mutations have defined two regions of nut, boxA and boxB (Friedman & Gottesman, 1983). The boxA site is conserved in sequence among other phages and E. coli whereas the box B site is unique for each phage type and is thought to be the N recognition site (Lazinski, et al., 1989) (See Figure 1).