FIG. 5.

Proposed model of MucA-mediated coordination of alginate production and T3SS expression. MucA is a transmembrane protein, with its cytoplasmic portion binding and inhibiting the sigma factor AlgU. Upon sensing of certain environmental stress signals by the periplasmic MucB, it signals MucA through the periplasmic domain to release the bound AlgU. Free AlgU is required for the expression of downstream transcriptional activators AlgP, AlgQ, AlgB, and AlgR, all of which contribute to the optimal expression of the algD operon, encoding enzymes for the synthesis of alginate. AlgR, on the other hand, also activates downstream genes which are responsible for the suppression of the type III secretion genes.