FIG 8.

Updated model for regulation of expression of the C. perfringens nanI sialidase gene. (A) In the absence of sialic acid, NanR represses nanI expression. (B) In the presence of sialic acid, ManNAc-6P is generated from enzymes (blue boxes) encoded by the nan operon (gray arrows); ManNAc-6P then binds to and inactivates NanR to relieve NanR-mediated repression of nanI expression. This leads to strong nanI expression in the presence of low sialic acid levels. However, in the presence of high sialic acid levels, another, unidentified regulator (green box) also represses nanI expression. (C) In the absence of sialic acid but the presence of high glucose levels, both NanR and a second (unidentified) regulator (yellow box), possibly the same non-NanR regulator shown in panel B, repress nanI expression. The second repressor also functions in the presence of high glucose levels but low sialic acid levels.