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Effects of external ammonium on biofilm development. a, Addition of external ammonium (red shading, 1 mM) represses expression from the PnasA-YFP reporter (black), but does not affect expression from a constitutive reporter (Phyperspank-CFP + 1 mM IPTG, gray). b, Removal of external ammonium (red shading, 13 mM) causes halting of colony growth.
