Changes in the mycobacterial transcriptome in response to nitrogen stress. Significant gene expression response is observed upon external nitrogen depletion. Very few genes display changes in expression until external depletion, where approximately 1000 genes are differentially expressed in nitrogen limiting and not nitrogen excess environments. Up-regulated genes are represented by black bars and down regulated genes represented by grey bars. Microarray expression data was obtained from three independent biological replicates. The concentration of ammonium (mM) in the external medium is represented by a grey dashed line with closed triangles.