The N Protein of λ: Evidence Bearing on Transcription Termination, Polarity and the Alteration of E. coli RNA Polymerase

Abstract

INTRODUCTION
In the context of this volume it is appropriate to review current information concerning the role of the N protein as a positive regulator of bacteriophage λ. Functional and genetic evidence on this key regulator have led us to postulate that it works by modifying Escherichia coli RNA polymerase so that its ability to terminate transcription is altered (Franklin 1974; see also Friedman, Wilgus and Mural 1973; Adhya, Gottesman and de Crombrugghe 1974). Implicit is the corollary that termination is a function, at least in part, of polymerase itself. Additional evidence makes it likely that terminations near gene ends, provoked by the absence of translation, account for transcriptional polarity in E. coli.

RESULTS AND DISCUSSION
N as Antiterminator of λ Transcription at Specific Targets
The protein product of λ’s N gene is required for the expression of all λ genes except imm, cro and N itself. Early λ genes function as components of two operons only: the N operon, including N as its first structural gene, is transcribed leftwards from the p_L promoter; the cro operon, with cro as its first structural gene, is transcribed rightwards from the p_R promoter (Figure 1). Transcription initiated at p_L or p_R by E. coli RNA polymerase (Takeda et al. 1969) is terminated, in the absence of the N protein, just beyond N in the region t_L and just beyond cro in the region t_R1, as well as in the region t_R2, between P and Q. The role of N protein in λ...