Interfakultäres Institut für Mikrobiologie und Infektionsmedizin

Nitrogen metabolism and its regulation

Nitrogen occurs in all organisms and is one of the four (N, P, C,S) fundamental elements required for the production of nearly all macromolecules of the cell for example DNA, RNA, proteins, peptidoglycan as well as primary and secondary metabolites. Generally, 1 g of bacterial cell mass comprises of 50 mg of nitrogen which reflects high need of this element during growth. In particular, nitrogen is one of the bioelements which is only poorly accessible in the natural environment. On the other hand, if nitrogen-containing compounds are becoming available for the non-motile streptomyces, they have to react promptly on rapid changes. Therefore, streptomyces require complex machinery regulating the nitrogen metabolism under different growth conditions as well as a number of versatile enzymes ensuring efficient assimilation of nitrogen.

Work in our group focuses on the nitrogen control in antibiotic producing actinomyces:

Streptomyces coelicolor M145, a model organism for Streptomyces

Streptomyces tsukubaensis NRRL 18488 , a tacrolimus producer

Microbispora sp. ATCC-PTA-5024, a lantibiotic (NAI-107) producer

Nitrogen assimilation

Ammonium is almost always the preferred nitrogen source, as it can be assimilated directly into glutamine and glutamate, the key donors for biosynthetic reactions and is therefore the energetically less expensive substrate. If in excess, ammonium can easy diffuse through the membrane whereas under the N-limitation it has to be actively transported by AmtB transporter. Other N-sources must first be degraded to ammonium or must be reduced. Nitrate is transported into the cell by NarK transporter and then reduced to nitrite by the nitrate reductase NasA, later subsequently to ammonium by nitrite reductase NirBD. Ammonium is introduced into the metabolism either by the glutamate dehydrogenase GDH (encoded by gdhA) or the glutamine synthetases GSI/GSII and glutamate synthase GOGAT, encoded by glnA, glnII and gltBD, respectively.
Whereas the GDH works under conditions of high nitrogen supply, the GS/GOGAT pathway is active under N-limiting conditions. Nitrogen assimilation in actinobacteria is strictly controlled on transcriptional as well as post-translational level.
Three transcriptional regulators: GlnR – global nitrogen regulator, GlnRII- secondary nitrogen regulator and NnaR –nitrate/nitrite assimilation regulator, govern the expression of over 20 described nitrogen genes in S. coelicolor. Post-translational regulation is ensured by: GlnK –PII signal protein, GlnE –glutamine synthetase I – adenylyltransferase and GlnD- PII protein - adenylyltransferase.

Currently we are studying following topics

Streptomyces coelicolor M145:

• Post-translational modifications of GlnR and their role in the GlnR regulatory function

• Role of GlnRII and GSII in the N-assimilation
• Interactions of the P -, N - and C – metabolism
• Function of GlnA-like enzymes in the nitrogen metabolism

Microbispora sp. ATCC-PTA-5024:

• Characterization of the GlnR-regulon

• Influence of nitrogen metabolism on the NAI-107 production

Streptomyces tsukubaensis NRRL 18488:

• Characterization and improvement of the nitrate/nitrite metabolism

• Characterization of the GlnR-regulon, nitrogen metabolism and it influence on tacrolimus production

• Optimization of the nitrogen containing precursor supply in tacrolimus biosynthesis

N-Group Members

Former members of the N-Group

• M.Sc Marco Jakobi
• Juliane Roth
• M.Sc Yousra Ahmed
• Dr. Rafat Amin