GSE211141
Role of the extracytoplasmic function sigma factor SigE in the stringent response of Mycobacterium tuberculosis
Bacteria respond to nutrient starvation implementing the stringent response, a stress signalling system resulting in metabolic remodelling leading to decreased growth rate and energy requirements. A well-characterized model of stringent response in Mycobacterium tuberculosis is that one induced by growth in low phosphate. The extracytoplasmic function (ECF) sigma factor SigE was previously suggested having a key role in the activation of stringent response. In this study we challenge this hypothesis by analyzing the temporal dynamics of the transcriptional response of a sigE mutant and its wild-type parental strain to low phosphate using RNA sequencing. We found that both strains responded to low phosphate with a typical stringent response trait, including the downregulation of genes encoding ribosomal proteins and RNA polymerase. We also observed transcriptional changes that support the occurring of an energetics imbalance, compensated by a reduced activity of the electron transport chain, decreased export of protons and a remodelling of central metabolism. The most striking difference between the two strains was the induction in the sigE mutant of several stress-related genes. In particular, the genes encoding the ECF sigma factor SigH and the transcriptional regulator WhiB6. Since both proteins respond to redox unbalances, their induction suggests that the sigE mutant is not able to maintain redox homeostasis in response to the energetics imbalance induced by low phosphate. In conclusion, our data suggest that SigE is not directly involved in initiating stringent response, but in protecting the cell from stress consequent to the exposure to low phosphate and activation of stringent response. Overall design: Comparative gene expression profiling analysis over time of M. tuberculosis of RNA-seq data for wild-type H37Rv strain (WT) and its isogenic sigE null mutant ST28 (MU) in which SigE is rendered non-functional. Triplicate WT and MU cultures grown in phosphate-rich substrate were washed and re-suspended in low-phosphate substrate at 0 hours. RNA extraction was performed at 0 hours (high phosphate) and 6, 12, and 24 hours (low phosphate).