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Secretion of theEscherichia coli toxin colicin V was previously determined to be iron regulated via the Fur (ferric uptake regulator) protein, based on studies infur mutants. The iron dependence of transcription and expression ofcvaA , which encodes a transporter accessory protein, andcvi , encoding the colicin V immunity protein, was assessed under conditions of iron excess or depletion. Immunoblots showed that production of both Cvi and CvaA is iron dependent. The iron-dependent transcriptional start forcvaA identified by primer extension and S1 nuclease analysis, P1, lies 320 bp upstream of the translational start and is associated with a newly identified Fur binding site. β-Galactosidase activity in transcriptionallacZ fusions with the P1 promoter alone is higher than with downstream sequences present and is induced 10-fold by iron depletion. Including immediate downstream regions with P1 enhances activity from P1 even more but reduces the induction by iron depletion fivefold. Including subsequent downstream sequences, however, down-modulates overall transcription from P1 almost fourfold. Deletion of a long stem-loop structure in this region alleviates the down-modulation by increasing transcription, indicating that the sequences or structure of this element may contribute to this down-regulation. Characterization of thecvi promoter by primer extension showed that it resides where predicted, about 50 bp upstream ofcvi associated with a previously identified Fur binding site. Thecvi promoter is also inducible by iron depletion. The modulating sequences fromcvaA were placed downstream of thecvi promoter to test their effects in transcriptional fusions of thecvi promoter tolacZ . The fusion results showed that these sequences also modulate transcription of thecvi promoter in a manner similar to that of thecvaA promoter. The potential for up- and down-regulation within the long untranslated region downstream of thecvaA promoter suggests a novel mechanism that fine-tunes expression of the colicin V secretion genes.

journal of bacteriology

Characterization of the cvaA and cvi Promoters of the Colicin V Export System: Iron-Dependent Transcription of cvaA Is Modulated by Downstream Sequences