coelicolor [55] or C. glutamicum [36]. It appeared as though NADP+-GDH in M. smegmatis had a constitutive ammonium assimilatory function under our experimental conditions. It was found, however, that the de-aminating activity of NADP+-GDH did change in response to nitrogen availability which suggests that the activity of NADP+-GDH in M. smegmatis is regulated

in a manner different to other Actinomycetes. It may be that an increase in glutamate CYC202 catabolism under these conditions could produce free ammonia required for essential glutamine production by GS. The high levels of NAD+-GDH aminating activity observed under all conditions of ammonium availability in M. smegmatis was unexpected as NAD+-GDH enzymes are presumed to be largely involved in glutamate catabolism. In addition, NAD+-GDH animating activity appeared to change in response to nitrogen availability which could indicate an important role in ammonium assimilation. In the absence of an initial upregulation of NAD+-GDH gene transcription under conditions of ammonium starvation, the observed increase in NAD+-GDH aminating activity might possibly be attributed to other control mechanisms, such as the GarA-pknG regulatory system. This type of regulation may also account for the observed decrease in NAD+-GDH aminating activity

upon exposure to an ammonium pulse. Transcription of msmeg_4699 and msmeg_6272 increased after prolonged exposure to nitrogen starvation (2 to 4 hrs ammonium starvation), which similarly to GS, could contribute to the maintenance this website of elevated levels of activity under those conditions. An inherent limitation of this study is that cell free extracts were used in enzyme activity assays which may possibly contain enzymes/proteins other than the glutamate dehydrogenases that could utilize NAD(P)H as co-factors and therefore confound GDH assay results. However, since whole cell lysates almost have been utilized successfully in previous studies [10, 37, 56], the possibility that the observed changes in enzyme activity are true

physiological responses to nitrogen availability should not be disregarded. From our results, it would appear that there are differences in the roles that the various GDH enzymes play in M. smegmatis and in other related organisms. There are also differences between the mycobacteria. The slow growing pathogenic mycobacteria such as M. tuberculosis and M. bovis do not appear to have an NADP+-GDH, however both genomes do encode for an NAD+-GDH which share a 81% and 82% amino acid identity with MSMEG_4699 respectively. The results obtained from our study imply that NAD+-GDH may play a previously unpredicted and find more potentially important nitrogen assimilatory role in these pathogenic species.