Model for σB-dependent translational activation. Schematic of a polycistronic operon containing a σA-dependent promoter (PA), σB-dependent promoter (PB), coding sequences, and a terminator. (A) In the absence of σB, transcription from PA produces a polycistronic mRNA molecule containing secondary structures that translationally repress the σB-dependent open reading frame (red) by sequestering its Shine–Dalgarno sequence (blue) and start codon (green). (B) PB becomes transcriptionally active upon σB induction, generating an mRNA isoform with an alternative transcription start site (magenta). Without the sequences necessary to form stable secondary structures, these transcripts can recruit ribosomes more efficiently to facilitate greater protein expression.
