Nitrogen metabolism in Streptomyces coelicolor: transcriptional and post-translational regulation

Glutamine synthetases (GS) are key enzymes of nitrogen metabolism. Most bacteria contain only one type of GS enzyme encoded by glnA. Streptomyces coelicolor, the model organism for Gram-positive streptomycetes, however is characterized by two functional GS (glnA, glnII) involved in nitrogen assimilation. In addition, three GS-like genes were identified which do not exhibit GS enzyme activity. The control of nitrogen assimilation and metabolism is mediated by transcriptional and post-translational regulation systems. The OmpR-like regulators GlnR and GlnRII are involved in transcriptional control of important nitrogen metabolism genes (glnA, glnII, amtB, glnK, glnD). Although GlnR and GlnRII share identical binding regions, their physiological impact is different. GSI activity is modulated post-translationally by the adenylyltransferase GlnE in response to the nitrogen concentration whereas no post-translational modifications of GSII are known. The PII/GlnD system also responds to changes in nitrogen conditions. The adenylyltransferase GlnD, which resembles the uridylyltransferase of Enterobacteriaceae, modifies PII under low-nitrogen conditions. Furthermore, PII is processed at its N-terminus in response to an ammonium shock. Apparently the function of the PII protein of S. coelicolor is different from that of the PII proteins of Enterobacteriaceae.