Promoter characterization of relZ-bifunctional (pp)pGpp synthetase in mycobacteria

The second messenger guanosine 3',5'-bis(diphosphate)/guanosine tetraphosphate (ppGpp) and guanosine 3'-diphosphate 5'-triphosphate/guanosine pentaphosphate (pppGpp) ((p)ppGpp) has been shown to be crucial for the survival of mycobacteria under hostile conditions. Unexpectedly, deletion of primary (p)ppGpp synthetase-Rel did not completely diminish (p)ppGpp levels leading to the discovery of novel bifunctional enzyme-RelZ, which displayed guanosine 5'-monophosphate,3'-diphosphate (pGpp), ppGpp, and pppGpp ((pp)pGpp) synthesis and RNAseHII activity. What conditions does it express itself under, and does it work in concert with Rel? The regulation of its transcription and whether the Rel enzyme plays a role in such regulation remain unclear. In this article, we have studied relZ promoter and compared its activity with rel promoter in different growth conditions. We observed that the promoter activity of relZ was constitutive; it is weaker than rel promoter, lies within 200 bp upstream of translation-start site, and it increased under carbon starvation. Furthermore, the promoter activity of relZ was compromised in the rel-knockout strain in the stationary phase. Our study unveils the dynamic regulation of relZ promoter activity by SigA and SigB sigma factors in different growth phases in mycobacteria. Importantly, elucidating the regulatory network of RelZ would enable the development of the targeted interventions for treating mycobacterial infections.