Model of SiaR and CRP regulation of the nan and siaPT operons. A. In the absence of sialic acid and cAMP, SiaR is bound to its operator and expression of the nan and siaPT operons is repressed. B. When cAMP is present, CRP binds to its operator and is able to activate the siaPT operon, but not the nan operon. C. When both GlcN-6P and cAMP are present, SiaR and CRP are active and interact to form a complex that activates both the nan and siaPT operons. D. In helical phasing experiments, insertion of one half-turn in between the SiaR and CRP operators prevents the regulators from interacting and thus maximal activation of the nan operon is not achieved.