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When translation of trp mRNA is terminated by a nonsense codon or by antibiotics like chloramphenicol, the amount of the mRNA distal to the blocked ribosomes is found at much lower levels ("polarity"). Polarity is alleviated when the trp mRNA is formed as part of a long transcript from the phage lambda promoter PL (Segawa and Imamoto, 1974; Franklin, 1974); but the relief of polarity is itself largely dependent on the lambda protein N. In a phage that joins the trp operon segment (trpD, C, B A) to a point distal to the N gene, lacking the tL site, synthesis of trp mRNA starting at the PL promoter continues even when translation is generally inhibited by chloramphenicol, but in the absence of functional N gene product synthesis of the mRNA can be blocked by the antibiotic. Unexpectedly, in the absence of N function, even when translation is occurring, weak termination of transcription occurs at some sites in the translocated trp operon.

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When the trp operon is translocated into the early region of lambda phage, transcription originated at the PL promotor is known to be modified by function of the N gene product so that transcription of the operon continues when translation is blocked by nonsense mutations or by ribosomal antibiotics. When N function is deficient in a phage that joins the trp operon to a point distal to the N gene, deleting the tL site, nonsense mutations (Franklin, 1974) or chloramphenicol (Nakamura et al., accompanying paper) again block transcription of the bacterial operon. However, here we report that transcription over about the first 800 nucleotide pairs starting from the PL promotor of the N-trp operon is still insensitive to chloramphenicol even in the absence of N function. The region covers the full N gene and the initial bit of the trp operon.