Substrate dependent homeostatic control of c-di-AMP synthase (MsDisA) and hydrolase (MsPDE) from Mycobacterium smegmatis

Cyclic-di-nucleotide based secondary messengers regulate various physiological processes including the stress responses in bacteria. In the past decade, cyclic diadenosine monophosphate (c-di-AMP) has emerged as a crucial second messenger, implicated in fatty acid metabolism, antibiotic resistance, biofilm formation, virulence, DNA repair, ion homeostasis, sporulation etc. The level of c-di-AMP is maintained in the cell by the action of two opposing enzymes, namely diadenylate cyclase (DAC) and phosphodiesterase (PDE). In mycobacteria, this molecule is essential for its regulatory role in bacterial physiology and host-pathogen interactions. However, such modulation of c-di-AMP remains to be explored in Mycobacterium smegmatis. Here, we systematically characterised the c-di-AMP synthase (MsDisA) and a hydrolase (MsPDE) from M. smegmatis at different pH and osmolytic conditions in vitro. Our biochemical assays show that the MsDisA activity is enhanced during the alkaline stress and c-di-AMP is readily produced without any intermediates. At pH 9.4, the MsDisA promoter activity in vivo increases significantly, strengthening this observation. However, under physiological conditions, the activity of MsDisA was moderate with the formation of intermediates. To get further insights into the structural characteristics, we determined the cryo-EM structure of the MsDisA, revealing some interesting features. Biochemical analysis of individual domains shows that the N-terminal minimal region alone can form a functional octamer. Altogether, our results reveal the biochemical and structural regulation of mycobacterial c-di-AMP in response to various environmental stress.