FIGURE 4.

Mutually exclusive, alternative stem-and-loop structures that form in the his leader transcript as part of the attenuation process. Letters designate segments of the leader transcript that base pair to form secondary structures. Ribosomes labeled lines indicate the nucleotides masked by a ribosome stopped at a codon. The model for his attenuation is described in the text. (A) Transcription termination configuration caused by translation of the leader transcript to the UAG stop codon; (B) readthrough transcription configuration caused by ribosome stalling at the fourth histidine codon in the leader transcript; (C) transcription termination configuration caused by ribosome stalling at the Gln codon in the leader transcript. Configuration C will also occur in the absence of translation of the leader transcript or after rapid dissociation of the ribosome at the translation stop codon. Configuration C also indicates the pause site (arrowhead marked with P) where RNA polymerase temporarily halts elongation following synthesis of A:B (57, 59, 146). The role of pausing in synchronizing the attenuation mechanism is described in the text. Figure modified from reference (127).