Identification of a RelA/SpoT Homolog and Its Possible Role in the Accumulation of Astaxanthin in Haematococcus pluvialis

Abstract

A RelA/SpoT homolog, HpRSH, was identified in Haematococcus pluvialis. HpRSH was found to catalyze Mg2+-dependent guanosine tetraphosphate (ppGpp) synthesis and Mn2+-dependent ppGpp hydrolysis, respectively. The transcription of HpRSH was significantly upregulated by environmental stresses, such as darkness, high light, nitrogen limitation, and salinity stress. The intracellular ppGpp level was also increased when exposed to these stresses. In addition, the classical initiator of stringent response, serine hydroxamate (SHX), was found to upregulate the transcription of HpRSH and increase the level of ppGpp. Moreover, stringent response induced by SHX or environmental stresses was proven to induce the accumulation of astaxanthin. These results indicated that stringent response regulatory system involved in the regulation of astaxanthin biosynthesis in H. pluvialis. Furthermore, stringent response was unable to induce astaxanthin accumulation under dark condition. This result implied that stringent response may regulate astaxanthin biosynthesis in a light-dependent manner.

Metabolomics and proteomics analyses revealed mechanistic insights on the antimicrobial activity of epigallocatechin gallate against Streptococcus suis

“…RelA/SpoT enzymes control bacterial physiology through the synthesis and degradation of the nucleotide alarmone (p)ppGpp ( Kurata et al., 2021 ). When, bacterial cells encounter environmental stresses, such as cold, nitrogen limitation, salinity stress, and astaxanthin exposure, RelA/SpoT expression is significantly up-regulated to adapt to the new environment ( Jin et al., 2022 ). Therefore, on the basis of these observations, we speculate that EGCG may regulate the expression of ABC transporters to allow EGCG influx into SC19 cells and affect the normal metabolic activity of these channels.…”

New perspectives on the molecular mechanisms of stress signalling by the nucleotide guanosine tetraphosphate (ppGpp), an emerging regulator of photosynthesis in plants and algae

“…This may therefore be one of the processes by which dark‐induced stromal Ca 2+ transients can influence chloroplast function (Rocha et al ., 2014 ). Cyanobacteria and algae also accumulate (p)ppGpp in the dark but do not have RSH enzymes with EF‐hand domains for Ca 2+ binding (Hood et al ., 2016 ; Puszynska & O'Shea, 2017 ; Jin et al ., 2022 ). This suggests that dark‐induced (p)ppGpp signalling is widespread in photosynthetic organisms, although the activation of RSH enzymes must occur via distinct mechanisms.…”