Figure 4. Repression of SucA by endogenous GlnZ.

(A) GlnZ post-transcriptionally represses the expression of SucA in Salmonella during growth on glutamine (Gln) as the nitrogen source. Salmonella wild-type (WT) and ΔglnZ strains were grown to exponential phase (OD₆₀₀ ~0.5) in MOPS minimal medium containing 0.2% glucose as the carbon source and different nitrogen sources; 0.1% ammonium, 0.01% ammonium, 5 mM Gln, and 5 mM Gln plus 0.01% ammonium. (B) GlnZ and Nac independently repress the expression of SucA in Escherichia coli during growth on Gln as the nitrogen source. E. coli WT, ΔglnZ, Δnac, and ΔglnZΔnac strains were grown to exponential phase (OD₆₀₀ ~0.5) in MOPS minimal medium containing 0.2% glucose as the carbon source and either 0.1% ammonium or 5 mM Gln as the nitrogen source. The expression of SucA and glutamine synthetase (GS) was analyzed by western blots (upper panels), and that of GlnZ was analyzed by northern blots (bottom panels). GroEL served as a loading control for western blots; 5S rRNA for northern blots. The expression levels of SucA relative to GroEL were normalized to that of WT strain grown on Gln, and the standard deviation was calculated from biological replicates (n>3). Statistical significance was calculated using one-way ANOVA and denoted as *p<0.05.