Characterization of a bifunctional enzyme with (p)ppGpp-hydrolase/synthase activity in Leptospira interrogans

Alarmone Guanosine 5'-diphosphate (or 5'-triphosphate) 3'-diphosphate [(p)ppGpp] is the key component that globally regulates stringent control in bacteria. There are two homologous enzymes, RelA and SpoT in Escherichia coli, which are responsible for fluctuations in (p)ppGpp concentration inside the cell, whereas there exists only a single RelA/SpoT enzyme in Gram-positive bacteria. We have identified a bifunctional enzyme with (p)ppGpp-hydrolase/synthase activity in Leptospira interrogans. We show that the relLin gene (LA_3085) encodes a protein that fully complements the relA/spoT double mutants in E. coli. The protein functions as a (p)ppGpp degradase as well as a (p)ppGpp synthase when the cells encounter amino acid stress and deprivation of carbon sources. N-terminus HD and RSD domains of relLin (relLinN ) were observed to restore growth of double mutants of E. coli. Finally, We demonstrate that purified RelLin and RelLinN show high (p)ppGpp synthesis activity in vitro. Taken together, our results suggest that L. interrogans contain a single Rel-like bifunctional protein, RelLin , which plays an important role in maintaining the basal level of (p)ppGpp in the cell potentially contributing to the regulation of bacterial stress response.