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Background  The cell wall of pathogenic mycobacteria is known to possess poly-L-glutamine (PLG) layer. PLG synthesis has been directly linked to glutamine synthetase (GS) enzyme.  glnA1  gene encodes for GS enzyme in mycobacteria. PLG layer is absent in cell wall of avirulent  Mycobacterium smegmatis , although  M. smegmatis  strain expressing GS enzyme of pathogenic mycobacteria can synthesize PLG layer in the cell wall. The role of GS enzyme has been extensively studied in  Mycobacterium tuberculosis , however, little is known about GS enzyme in other mycobacterial species.  Mycobacterium bovis,  as an intracellular pathogen encounters nitrogen stress inside macrophages, thus it has developed nitrogen assimilatory pathways to survive in adverse conditions. We have investigated the expression and activity of  M. bovis  GS in response to nitrogen availability and effect on synthesis of PLG layer in the cell wall.  M. smegmatis  was used as a model to study the behaviour of  glnA1  locus of  M. bovis.Results  We observed that GS expression and activity decreased significantly in high nitrogen grown conditions. In high nitrogen conditions, the amount of PLG in cell wall was drastically reduced (below detectable limits) as compared to low nitrogen condition in  M. bovis  and in  M. smegmatis  strain complemented with  M. bovis glnA1 . Additionally, biofilm formation by  M. smegmatis  strain complemented with  M. bovis glnA1  was increased than the wild type  M. smegmatis  strain.    Conclusions  The physiological regulation of GS in  M. bovis  was found to be similar to that reported in other mycobacteria but this data revealed that PLG synthesis in the cell wall of pathogenic mycobacteria occurs only in nitrogen limiting conditions and on the contrary high nitrogen conditions inhibit PLG synthesis. This indicates that PLG synthesis may be a form of nitrogen assimilatory pathway during ammonium starvation in virulent mycobacteria. Also, we have found that  M. smegmatis  complemented with  M. bovis glnA1  was more efficient in biofilm formation than the wild type strain. This indicates that PLG layer favors biofilm formation. This study demonstrate that the nitrogen availability not only regulates GS expression and activity in  M. bovis  but also affects cell surface properties by modulating synthesis of PLG.

Poly-L-glutamate/glutamine synthesis in the cell wall of Mycobacterium bovisis regulated in response to nitrogen availability